Abstract:

An image forming apparatus includes an image forming part, a fixer
including a heating member and a pressure member, a reverse transport
part to forward a recording medium transported from the fixer to the
image forming part, a toner image sensor to detect presence of a toner
image on the recording medium, and a controller. When a glossy image
formation mode is selected and no toner image is detected on the
recording medium, the image forming part does not form a toner image on
the recording medium, the fixer performs a first heating and pressing
process and transports the recording medium to the reverse transport
part, the reverse transport part transports the recording medium to the
image forming part, the image forming part forms a toner image on the
recording medium transported from the reverse transport part, and the
fixer performs a second heating and pressing process and fixes the toner
image thereon.

Claims:

1. An image forming apparatus, comprising:an image forming part configured
to form a toner image on a recording medium;a fixer including a heating
member and a pressure member, configured to transport the recording
medium sandwiched therein, while fixing the toner image on the recording
medium with heat and pressure;a reverse transport part configured to
reverse and forward the recording medium transported from the fixer to
the image forming part;a toner image sensor configured to detect presence
of the toner image on the recording medium, anda controller configured to
control the image forming part, the fixer, and the reverse transport
part,wherein, when a glossy image formation mode is selected and no toner
image is detected on the recording medium, the image forming part does
not form a toner image on the recording medium, the fixer performs a
first heating and pressing process and transports the recording medium to
the reverse transport part, the reverse transport part transports the
recording medium to the image forming part, the image forming part forms
a toner image on the recording medium transported from the reverse
transport part, and the fixer performs a second heating and pressing
process and fixes the toner image on the recording medium.

2. The image forming apparatus according to claim 1, wherein the
controller sets a heating temperature in the first heating and pressing
process to a temperature higher than a heating temperature in the second
heating and pressing process.

3. The image forming apparatus according to claim 1, wherein the
controller sets a fixing linear speed in the first heating and pressing
process to a speed slower than a fixing linear speed in the second
heating and pressing process.

4. The image forming apparatus according to claim 1, further comprising:a
humidity sensor configured to detect humidity inside the image forming
apparatus;a first temperature sensor configured to detect a surface
temperature of the heating member; anda second temperature sensor
configured to detect a surface temperature of the pressure member,wherein
the controller sets the surface temperatures of the heating member and
the pressure member to satisfy a
relation,Y-Z≦(-6.7.times.X)-150wherein Y is the surface
temperature of the heating member in degrees Centigrade, Z is the surface
temperature of the pressure member in degrees Centigrade, and X is the
humidity detected by the humidity sensor in grams per cubic meter.

5. The image forming apparatus according to claim 1, further comprising:a
humidity sensor configured to detect humidity inside the image forming
apparatus;a first temperature sensor configured to detect a surface
temperature of the heating member; anda second temperature sensor
configured to detect a surface temperature of the pressure
member,wherein, when the recording medium has a basic weight of 80
g/m2 or smaller, the controller sets the surface temperatures of the
heating member and the pressure member to satisfy a
relation,Y-Z≦{(-6.7.times.X)-150}×0.9wherein Y is the
surface temperature of the heating member in degrees Centigrade, Z is the
surface temperature of the pressure member in degrees Centigrade, and X
is the humidity detected by the humidity sensor in grams per cubic meter.

6. The image forming apparatus according to claim 1, wherein at least one
of the heating member and the pressure member is an endless belt looped
around a plurality of rollers.

7. The image forming apparatus according to claim 1, wherein at least one
of the heating member and the pressure member is a free nip belt.

8. The image forming apparatus according to claim 1, wherein at least one
of the heating member and the pressure member is heated by
electromagnetic induction heating.

9. The image forming apparatus according to claim 1, wherein, when a
double-side printing mode to form toner images on both a first side and a
second side of the recording medium is selected, controlled by the
controller, the image forming part forms a toner image on the first side
of the recording medium, the reverse transport part reverses and forwards
the recording medium having the toner image fixed on the first side
thereof to the image forming part, and the image forming part forms a
toner image on the second side of the recording medium.

10. The image forming apparatus according to claim 1, wherein the toner
image is formed with a toner produced through a method comprising:forming
an oil-base dispersion liquid by dissolving or dispersing, in an organic
solvent, a prepolymer including a modified polyester resin, a compound
with which the prepolymer performs at least one of elongation and
cross-linking, and a toner component including a colorant;inducing at
least one of elongation and cross-linking of the prepolymer, the
compound, and the toner component in an aqueous medium; andremoving the
organic solvent from the dispersion liquid,wherein the colorant dispersed
in toner particles has a number average particle diameter of 0.5 μm or
smaller as a dispersion particle diameter, and a ratio of colorant
particles having a particle diameter of 0.7 μm or greater is not
greater than 5% by number.

11. The image forming apparatus according to claim 10, wherein the
colorant included in the toner has a number average particle diameter of
not greater than 0.3 μm as a dispersion particle diameter, and a ratio
of colorant particles having a particle diameter of 0.5 μm or greater
is not greater than 10% by number.

12. The image forming apparatus according to claim 10, wherein the toner
particles have a weight average particle diameter and a number average
particle diameter that satisfy a
relation1.00.ltoreq.Dv/Dn≦1.20wherein Dv is the weight average
particle diameter, Dn is the number average particle diameter, and Dv/Dn
is particle diameter distribution.

13. The image forming apparatus according to claim 10, wherein the toner
particles have an average circularity within a range of from 0.900 to
0.960.

14. The image forming apparatus according to claim 10, wherein the
polyester resin included in the toner comprises an element that is
soluble in tetrahydrofuran and has a main peak of a molecular weight
distribution within a range from 2500 to 10,000, and a number average
molecular weight of the element soluble in tetrahydrofuran is within a
range from 2,500 to 50,000.

15. The image forming apparatus according to claim 10, wherein the
polyester resin included in the toner has a glass-transition temperature
within a range of from 40.degree. C. to 65.degree. C. and an acid number
within a range of from 1 KOH mg/g to 30 KOH mg/g.

16. The image forming apparatus according to claim 10, wherein a polyester
resin unreactive with amine is dissolved in the oil-base dispersion
liquid.

17. The image forming apparatus according to claim 10, wherein the toner
is mixed with a carrier and used in a developer.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This patent specification claims priority from Japanese Patent
Application No. 2007-103229, filed on Apr. 10, 2007 in the Japan Patent
Office, the entire contents of which are hereby incorporated by reference
herein.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention generally relates to an image forming
apparatus including a fixer.

[0004]2. Description of the Background Art

[0005]In various areas, an image forming apparatus, such as a copier, a
facsimile machine, a printer, etc., that employs an electronographic
method or an electrostatic recording method has been used. Such an image
forming apparatus includes an image carrier on which an electrostatic
latent image is formed, a developing device to develop the electrostatic
latent image with a developer, a transferer to transfer the image onto a
sheet of recording medium, and a fixer to fix the image on the sheet.

[0006]As an example, the fixer includes a pair of rollers facing each
other, and one of these rollers is used as a heating roller and the other
is used as a pressing roller to press the recording medium against the
heating roller. In such a configuration, the unfixed toner image is fixed
on the recording medium with heat from the heating roller and pressure
from the pressing roller while the recording medium passes through a nip
formed between these rollers.

[0007]Recently, a proportion of color images to total images output from
such image forming apparatuses has been increasing, and with it
increasing demand for higher quality images. One measure used to evaluate
color image quality is gloss level of a toner portion. While images of
lower gloss levels, for example, 15% or less, are generally preferred as
business documents, images of higher gloss level, for example, 20% or
greater, are generally preferred as printed materials including
catalogues and brochures. Therefore, in some cases, it is necessary to
use different types of image forming apparatuses depending on the purpose
of use of output images.

[0008]Generally, to produce images of higher gloss levels, a fixing
temperature and a fixing pressure are increased. However, in such a case,
moisture in the recording medium is likely to vaporize and swell, causing
spots in a toner image, which is a failure called blistering.

[0009]To prevent such blistering, fixers that perform preheating have been
proposed. For example, in one known method, a fixer preliminarily heats
the recording medium to 60 degrees Centigrade or higher when a coated
paper discriminating means determines that the recording medium is a
coated paper. In another known method, an upstream first fixer fixes a
unfixed toner image on a first side of a recording medium and a
downstream second fixer fixes a unfixed toner image on a second side of
the recording medium. A heating temperature of the first fixer is lower
than that of the second fixer, and the unfixed toner images on both sides
of the recording medium are fixed in one fixing process. Further, in
another known method, a plurality of fixing nips is formed with respect
to a fixing belt, so that fluctuation in a belt transportation speed is
prevented to achieve a uniform gloss level.

[0010]It is common to produce the toner for an electronograph through a
kneading and pulverization method in which a thermoplastic resin is
dissolved and kneaded together with a colorant and, if required, a
releasing agent such as wax and/or a charge controller. This mixture is
then pulverized and sorted. To surfaces of the toner particles thus
obtained, inorganic or organic fine particles are added, as required, to
improve fluidity and/or facilitate cleaning.

[0011]A toner produced through a known kneading and pulverization method
generally has an indefinite form, a relatively broad particle
distribution, relatively low fluidity, and relatively low transfer
properties. The toner requires a relatively large amount of energy to be
fixed, and a charge amount is rather uneven among the toner particles,
and thus its charge stability is relatively low. Further, the quality of
the images produced with such toner is inadequate.

[0012]As another method to produce toner, polymerization methods have been
proposed. The polymerization methods do not include kneading and
pulverization processes, and thus energy, production time, and cost can
be reduced. Further, such a polymerized toner produced through
polymerization has a particle distribution narrower than that of the
toner produced through the pulverization method. Moreover, the fluidity
of the polymerized toner can be enhanced significantly because
capsulation of wax is relatively easy, and thus roundish toner particles
can be produced relatively easily.

[0013]However, properties of such a polymerized toner are inadequate even
though the polymerized toner particles are generally rounder than the
pulverized toner particles because surface tension acts in the
polymerization process. Further, controlling the ultimate shape of toner
particles (deformation) is difficult in the polymerization method. As for
the charge stability and the transfer properties, the polymerization
methods are advantageous.

[0014]Among the polymerization methods, although a suspension
polymerization method is widely used, in this method, a monomer for a
binder (binder resin) is limited to styrene monomer, acrylic monomer,
etc., which are harmful to humans. As a result, the toner produced
through the suspension polymerization method includes such toxic
components and is thus not environmentally friendly.

[0015]Further, because wax is included inside the toner particles in the
case of the suspension polymerized method, the wax is less likely to
appear on the surfaces of the toner particles when the toner is used,
compared with the pulverized toner in which wax exist on the surfaces of
the particles. Therefore, although the toner is less likely to adhere to
the photoreceptor, the suspension polymerized toner has a lower fixing
efficiency, thus consuming a greater amount of power for fixing than the
pulverized toner does.

[0016]Further, in the case of the polymerized toner, when wax is increased
in amount and/or dispersion particle diameter so as to enhance fixing,
transparency of color image is impaired, and thus such a toner is not
suitable for OHP films used in presentations.

[0017]Another method to produce a polymerized toner is an emulsion
polymerization method that can deform particles to a certain level. In
the emulsion polymerization method, the monomer to be used is also
limited to styrene monomer, and removing an unreacted monomer, an
emulsifier, and a dispersant from toner particles completely is
difficult, and thus the toner is not environmentally friendly.

[0018]Yet another method to produce a polymerized toner is a dissolution
suspension method that can use a polyester resin that is fixable at a
lower temperature. However, because a component having a relatively large
molecular weight is used in a process to dissolve or disperse such a
resin and a colorant, viscosity of the dissolution or dispersion liquid
is increased, and productivity is decreased.

[0019]Further, in the dissolution suspension method, toner particles are
spherical and surfaces thereof are uneven so that the toner particles can
be better cleaned. However, the shape of such toner particles is
irregular, and charge stability thereof is relatively low. Moreover,
durability and a release are relatively poor, and thus the quality
thereof is inadequate.

[0020]A dry toner that has a practical sphericity within a range of from
0.90 to 1.00 has been proposed to improve fluidity, fixing at a low
temperature (low temperature fixing properties), and resistance to hot
offset. Hot offset is a phenomenon that occurs when toner is overheated,
reducing cohesion among toner particles below that to a fixing roller or
to paper. In such a condition, a toner layer separates, and toner in a
resultant image will be partly absent.

[0021]Such a proposed toner uses a toner binder manufactured through an
extending reaction of a urethane-modified polyester. Further, another dry
toner has been proposed that has higher powder fluidity and transfer
properties when the toner has a smaller particle diameter. This toner
also excels in thermal and storage stability, the low temperature fixing
properties, and resistance to hot offset. Methods to manufacture these
toners include a molecular weight increasing process in which a polyester
prepolymer containing an isocyanate group and amine perform polyaddition
reaction in an aqueous medium.

[0022]However, in the polymerized toner obtained through the method
described above, the colorant disperses unevenly, and transparency and
saturation of images produced with such a toner is lower. The images are
rather dark, particularly when images are formed on OHP films using such
a toner,

SUMMARY OF THE INVENTION

[0023]In view of the foregoing, in one illustrative embodiment of the
present invention, an image forming apparatus include an image forming
part configured to form a toner image on a recording medium, a fixer
including a heating member and a pressure member, configured to transport
the recording medium sandwiched therein, while fixing the toner image on
the recording medium with heat and pressure, a reverse transport part
configured to reverse and forward the recording medium transported from
the fixer to the image forming part, a toner image sensor configured to
detect presence of the toner image on the recording medium, and a
controller configured to control the image forming part, the fixer, and
the reverse transport part. When a glossy image formation mode is
selected and no toner image is detected on the recording medium, the
image forming part does not form a toner image, the fixer performs a
first heating and pressing process and transports the recording medium to
the reverse transport part, and the reverse transport part transports the
recording medium to the image forming part, the image forming part forms
a toner image on the recording medium transported from the reverse
transport part, and the fixer performs a second heating and pressing
process and fixes the toner image on the recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]A more complete appreciation of the disclosure and many of the
attendant advantages thereof will be readily obtained as the same becomes
better understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein:

[0025]FIG. 1 is a diagram illustrating a schematic configuration of an
image forming apparatus according to an illustrative embodiment of the
present invention;

[0026]FIG. 2 is a schematic cross section diagram illustrating a fixer
used in the image forming apparatus according to the illustrative
embodiment;

[0027]FIG. 3 is a graph illustrates relations among gloss level of images,
heating control temperature, and preheating;

[0028]FIG. 4 is a graph illustrates relations between gloss level of
images and heating control temperature when heating temperature in
preheating is changed;

[0029]FIG. 5 is a graph illustrates relations between gloss level of
images and heating control temperature when a linear speed in preheating
is changed;

[0030]FIG. 6 is a block diagram of a controller used in the image forming
apparatus according to an illustrative embodiment of the present
invention;

[0031]FIG. 7 is a schematic cross section diagram illustrating another
fixer according to the illustrative embodiment of the present invention;

[0032]FIG. 8 is a schematic cross section diagram illustrating another
fixer according to an illustrative embodiment of the present invention;

[0033]FIG. 9 is a graph illustrating a property of a toner used in the
image forming apparatus according to the illustrative embodiment;

[0034]FIG. 10 is a table illustrating properties of example toner binders
to be used in the image forming apparatus according to the illustrative
embodiment; and

[0035]FIG. 11 is a table illustrating properties of example toners to be
used in the image forming apparatus according to the illustrative
embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036]In describing preferred embodiments illustrated in the drawings,
specific terminology is employed for the sake of clarity. However, the
disclosure of this patent specification is not intended to be limited to
the specific terminology so selected, and it is to be understood that
each specific element includes all technical equivalents that operate in
a similar manner and achieve a similar result.

[0037]Referring now to the drawings, wherein like reference numerals
designate identical or corresponding parts throughout the several views
thereof, and particularly to FIG. 1, an image forming apparatus 20
according to an illustrative embodiment of the present invention is
described. As the image forming apparatus 20, a copier or printer that
can form full color images is used. Alternatively, the image forming
apparatus 20 may be a facsimile machine that can perform image forming
processes similar to those performed by the copier or printer according
to image signals externally received. It is to be noted examples of the
image forming apparatus 20 include a monochrome image forming apparatus.

[0038]Referring to FIG. 1, the image forming apparatus 20 includes an
image forming part that includes image forming units 21C, 21Y, 21M, and
21BK that form respective color images corresponding to original images,
and a transfer unit 22 facing the image forming units 21C, 21Y, 21M, and
21BK. An area in which the image forming units 21C, 21Y, 21M, and 21BK
face the transfer unit 22 is hereinafter referred to as a transfer area.
The image forming part forms a full color or monochrome image on a sheet
P of recording medium, such as transfer paper. Above the image forming
part, an optical writing device 29 is provided. The transfer unit 22
includes a transport belt 22a that rotates in a direction shown by arrow
B.

[0039]As the sheet P, the image forming apparatus 20 can use a sheet that
are commonly used in copying (standard sheet), an OHP (overhead
projector) film, a sheet having a ream weight of 90 kg such as a post
card, a thicker paper having a basic weight of 100 g/m2 or greater,
and a special sheet, such as an envelope, whose heat capacity is greater
than that of the standard sheet.

[0040]The image forming apparatus 20 further includes a manual feed tray
23, sheet cassettes 24a and 24b, a pair of registration rollers 30, and a
fixer 1. The sheet P is fed from one of the manual feed tray 23 and the
sheet cassettes 24a and 24b that are recording medium feed members, and
forwarded by the pair of registration rollers 30 to the transfer area in
timely manner to match image forming timings of the image forming units
21C, 21Y, 21M, and 21BK. After the images are transferred from the image
forming units 21C, 21Y, 21M, and 21BK and superimposed one on another on
the sheet P, the fixer 1 fixes the superimposed image on the sheet P.

[0041]The image forming apparatus 20 according to the present embodiment
can form images on both a first side and a second side of the sheet P in
a double-side printing mode and further includes a sheet reverse unit 31
and a sheet transport unit 32 to transport the sheet P in a direction
shown by arrow C. When the double-side printing mode is selected, after
the fixer 1 fixes an image on the first side of the sheet P, the sheet P
is reversed while transported through the sheet reverse unit 31 and the
sheet transport unit 32. The sheet P is further transported to the pair
of registration rollers 30 and then an image is formed on the second side
thereof.

[0043]The image forming unit 21C includes a photoreceptor drum 25C as an
electrostatic latent image carrier that rotates in a rotation direction
shown by arrow A. Around the photoreceptor drum 25C, a charging device
27C, a developing device 26C, and a cleaner 28C are provided along the
rotation direction shown by arrow A. The image forming unit 21C adopts a
known configuration in which, between the charging device 27C and the
developing device 26C, the photoreceptor drum 25C receives an exposure
light corresponding to a cyan image from the optical writing device 29.
Thus, an electrostatic latent image is formed on the photoreceptor drum
25C, and the electrostatic latent image is developed with a developer
into a cyan toner image.

[0044]As the developer, there are one-component developer and
two-component developer. The one-component developer include magnetic or
nonmagnetic toner, and the two-component developer include toner and
carrier.

[0045]It is to be noted that shape of the electrostatic latent image
carrier is not limited to a drum, and alternatively, a belt-shaped
electrostatic latent image carrier can be used. In the example shown in
FIG. 1, a footprint of the image forming apparatus 20 can be smaller
because the transfer unit 22 extends obliquely.

[0046]Similarly to the image forming unit 21C, each of the image forming
units 21Y, 21M, and 21BK includes one of photoreceptor drums 25Y, 25M,
and 25BK, one of developing devices 26Y, 26M, and 26BK, one of charging
device 27Y, 27M, and 27BK, and one of cleaners 28Y, 28M, and 28BK. The
image forming units 21C, 21Y, 21M, and 21BK have a similar configuration
except for only the color of the toner used therein, and thus
descriptions of the components thereof omitted.

[0047]The image forming apparatus 20 further includes a pair of discharge
rollers 33, a disparage port 34, a transport roller 35, and a toner image
detector 36 to determine presence of a toner image.

[0048]FIG. 2 illustrates an example of the fixer 1 that is a belt type. As
illustrated in FIG. 2, the fixer 1 includes an endless fixing belt 2 that
serves as a heating member and is looped around a heating roller 3 and a
fixing roller 4, a pressure roller 5 that serves as a pressure member and
faces the fixing roller 4 via the fixing belt 2, heaters 6 and 7, a first
temperature sensor 8a, and a second temperature sensor 8b. The heaters 6
and 7 are located inside the heating roller 3 and the pressure roller 5,
respectively. The fixing belt 2 moves in a direction shown by arrow E and
transports the sheet P in a direction shown by arrow D (sheet transport
direction). The first and second temperature sensors 8a and 8b are
thermistors, for example, and are located to face the fixing belt 2 and
the pressure roller 5 to detect temperatures thereof, respectively.

[0049]Biased by an elastic member, not shown, a tension roller 13 presses
the fixing belt 2 outward from to give the fixing belt 2 a predetermined
or desired tension.

[0050]The fixing roller 4 includes a metal core 9 and an elastic layer 10
that is a heat-resistant porous layer covering the metal core 9. The
fixing roller 4 is biased by an elastic member, not shown, in a direction
to press against the pressure roller 5. Reference numeral 12 indicates a
guide to guide the sheet P to a fixing nip 17 where the fixing roller 4
presses against the pressure roller 5 via the fixing belt 2.

[0051]It is to be noted that the heater 6 located inside the heating
roller 3 has a heating capacity larger than that of the heater 7 located
inside the pressure roller 5 because the fixing belt 2 has a heat
capacity smaller than that of the pressure roller 5 and a start-up time
in a cold start can be reduced when the pressure roller 5 receives heat
from an outer surface of the fixing belt 2 in addition to the heater 7.
In the present embodiment, the heating capacity of the heater 6 is 100 W
and the heating capacity of the heater 7 is 200 W when a voltage of 100 V
is applied thereto.

[0052]The heating roller 3, which drives the fixing belt 2 in cooperation
with the fixing roller 4 facing the pressure roller 5, is provided with a
heat source that heats an inner (back) surface of the fixing belt 2.
Further, a heat source to heat the outer surface of the fixing belt 2 is
provided inside the pressure roller 5. Because the fixing belt 2 has a
cubic volume and a heat capacity each smaller than those of a roller,
temperature thereof can be raised in a shorter time period, and thus the
fixer 1 has an advantage in that temperature can be raised in a shorter
time period compared with a configuration that uses only the heating
roller 3 and the pressure roller 5. Moreover, by including the heating
source in the pressure roller 5, both the outer and inner surfaces of the
fixing belt 2 can be heated so as to heat the fixing belt 2 more quickly.

[0053]The fixer 1 further includes a separation claw 11, an oil
application roller 14, a cleaning roller 15, and a humidity sensor 16.
The separation claw 11 is provided downstream of a fixing nip in the
sheet transport direction shown by arrow D, with a tip thereof pressed
against the outer surface of the fixing belt 2. When the sheet P adheres
to the outer surface of the fixing belt 2, the separation claw 11
separates the sheet P from the fixing belt 2 by entering therebetween
while the sheet P is transported to prevent the sheet P from being wound
around the fixing belt 2.

[0054]The fixer 1 adopts a configuration in which silicone oil is applied
to the fixing belt 2 to enhance a toner releasing property thereof. The
oil application roller 14 is used to apply a small amount of silicone oil
to the fixing belt 2. The oil application roller 14 includes a metal
core, a sponge-like foam member impregnated with silicone oil provided
over the metal core, and a semipermeable membrane rapped over the foam
member singly or doubly. Because the semipermeable membrane has
microscopic pores, silicone oil included in the foam member seeps out
therethrough, and thus the oil application roller 14 applies only a
relatively small amount of oil to an object facing the application roller
14. As the surface membrane of the oil application roller 14, a material
having a better toner releasing property is used so as to prevent toner
from firmly sticking on the surface of the oil application roller 14 when
toner adheres thereto, for example, during a paper jam. If toner firmly
sticks on the oil application roller 14 and fills in the microscopic
pores through which the oil soak out, oil application is inhibited. In
the present embodiment, a Gore-Tex (trademark) membrane is used for the
surface membrane as a material having a higher releasing property.

[0056]The sheet P having the toner image on the first side thereof is
transported to the fixer 1 in the direction shown by arrow D, and the
guide 12 guides the sheet P to the fixing nip 17. At the fixing nip 17,
the sheet P is sandwiched and pressed between the fixing roller 4 and the
pressure roller 5 and further heated by the fixing belt 2 so as to fix
the toner image thereon. The sheet P is then further transported in the
direction shown by arrow D by the fixing roller 4 that rotates to drive
the fixing belt 2 in the direction shown by arrow E.

[0057]In the present embodiment, a single-side printing mode M1 to form a
toner image on only one side of the sheet P and a double-side printing
mode M2 are selectable in the image forming apparatus 20. Further, two
modes are selectable regarding the sheet P itself: A glossy image mode M3
using the sheet P, which is a coated sheet in which a coating layer
including a resin covers a base and form a glossy toner image on the
coating layer; and a standard mode M4, to form a toner image on a
standard sheet.

[0059]Referring to FIGS. 1 and 2, in the standard single-side printing
mode, the optical writing device 29 directs exposure lights corresponding
to respective colors onto the photoreceptor drums 25C, 25Y, 25M, and 25BK
to form electrostatic latent images thereon. These electrostatic latent
images are developed into respective color images by the developing
devices 26C, 26Y, 26M, and 26BK, respectively. From one of the manual
feed tray 23 and the sheet cassettes 24a and 24b, a sheet P that is a
standard sheet is fed. The registration rollers 30 forward the sheet P to
the transport belt 22a in such a timely manner that the toner images are
transferred from the photoreceptor drums 25C, 25Y, 25M, and 25BK and
superimposed one on another on a first side of the sheet P while the
transport belt 22a transports the sheet P in the direction shown by arrow
B. After the toner image is thus formed thereon, the sheet P is
transported to the fixer 1 where the toner image is fixed with heat and
pressure by the fixing roller 4 and the pressure roller 5, and discharged
by the discharge rollers 33 through the disparage port 34.

[0060]In the standard double-side printing mode, processes are similar to
the processes performed in the standard single-side printing mode.
However, after the fixer 1 fixes the toner image formed on the first side
of the sheet P, the sheet P is not discharged through the discharge port
34, but a switch claw, not shown, changes a transport route of the sheet
P toward the sheet reverse unit 31.

[0061]After being reversed by the reverse unit 31 so that the first side,
on which the toner image is formed, faces up, the sheet P is transported
to the sheet transport unit 32, where the sheet P is further transported
in the direction shown by arrow C toward the registration rollers 30. The
registration rollers 30 forward the sheet P to the transfer unit 22 with
a second side thereof facing up and the first side thereof facing down in
such timely manner that a toner image is formed on the second side of the
sheet P. After the fixer 1 fixes the toner image on the second side of
sheet P, the sheet P is discharged through the discharge port 34.

[0062]The glossy image single-side printing mode is described below.

[0063]Because an amount of heat required to fix a glossy toner image on a
coated sheet is greater than that required to fix a toner image on the
standard sheet, in a method to fix the toner image with a fixer,
preheating is commonly performed before the sheet is transported to the
fixer.

[0064]For example, a fixer can include a first heating roller and a first
pressure roller for preheating, provided upstream of a second fixing
roller and a second pressure roller for fixing the toner image, and the
sheet is heated preliminarily while passing through between the first
heating roller and the first pressure roller. However, such a fixer is
larger and costs more because two pairs of heating rollers and pressure
rollers each are included therein instead of one.

[0065]Therefore, in the present embodiment, double-side printing process
is used to preheat the sheet P so as to produce excellent glossy toner
images without an additional preheating device.

[0066]In the present embodiment, when the user selects the glossy image
mode M3, the image forming apparatus 20 is set to the glossy image mode
M3. For example, the sheet cassette 24b contains the coated sheets each
including a coated layer, and the coated sheet is fed as the sheet P with
the coated side (first side) down from the sheet cassette 24b to the
registration rollers 30. While the sheet P is thus transported, the toner
image detector 36 shown in FIG. 1, provided between the transport roller
35 and the registration rollers 30, detects that no toner images are
formed on the first side of the sheet P.

[0067]When the glossy image mode M3 is set and the toner image detector 36
detects no toner images on the first side of the sheet P as described
above, the image forming units 21C, 21Y, 21M, and 21BK do not perform the
image forming processes and instead the sheet P is transported to the
fixer 1 by the registration rollers 30 and the transport belt 22a. In the
fixer 1, a first heating and pressing process of the sheet P is performed
by the fixing roller 4 and the pressure roller 5 as a preheating process,
and then the switch claw, not shown, forwards the sheet P to the reverse
unit 31.

[0068]The sheet P is reversed by the transport roller 35 so that the
coated side (first side) thereof turns up and then transported to the
registration rollers 30. The registration rollers 30 adjust the timing to
forward the sheet P to the image forming units 21C, 21Y, 21M, and 21BK.
When image forming conditions of the image forming units 21C, 21Y, 21M,
and 21BK are right, the sheet P is transported along the image forming
units 21C, 21Y, 21M, and 21BK so that respective color toner images are
transferred onto the coated side thereof and then forwarded to the fixer
1. The fixer 1 then performs a second heating and pressing process of the
sheet P to fix the toner image on the coated side thereof, and then the
sheet P is discharged through the discharge port 34.

[0069]In this case, as described above, the sheet P is preheated in the
first heating and pressing process before the toner image is fixed
thereon in the second heating and pressing process, and thus an excellent
glossy toner image can be produced, as is shown by an analysis of the
relation between heating control temperature of the fixer 1 and a gloss
level of the toner image that is now described below.

[0070]FIG. 3 is a graph illustrating a relation between a heating control
temperature of the fixer 1 and a gloss level of the toner image. The
graph shown in FIG. 3 is created based on experiments in which a toner
adhesion amount was set to 0.8 mg/cm2, an image forming linear speed
was set to 180 mm/s, and a sheet having a basic weight of 75 g/m2
was used as the sheet P.

[0071]In FIG. 3, a line 1 is results of a first experiment in which a
toner image was formed and fixed only on the first side of the sheet P
without preheating as in the standard single-side printing mode, which is
hereinafter also referred to as single pass.

[0072]A line 2 is results of a second experiment in which, after a toner
image was formed and fixed on the first side of the sheet P without
preheating, the sheet P was reversed through a sheet reverse path and
then the fixing process was performed on the second side thereof without
forming a toner image thereon, which is hereinafter also referred to as
double pass.

[0073]A line 3 is results of a third experiment according to the present
embodiment in which, after the first heating and pressing process
(preheating) was performed on the first side of the sheet P without
forming a toner image thereon, the sheet P was reversed through the sheet
reverse path, and a toner image was formed and fixed on the second side
in the second heating and pressing process (fixing process). The third
experiment is hereinafter also referred to as with preheating.

[0074]Referring to FIG. 3, in the first experiment whose results are shown
by the line 1, the gloss of the toner image was about 10%, and when the
fixing control temperature was about 190° C., a hot offset
phenomenon occurred on a surface of the toner image and the gloss level
was reduced.

[0075]In the second experiment whose results are shown by the line 2,
although the gloss was higher by about 2% than that in the experiment 1,
when the fixing control temperature was about 190° C., a hot
offset phenomenon occurred on a surface of the toner image and the gloss
level was reduced similarly to the first experiment.

[0076]In the third experiment according to the present embodiment whose
results are shown by the line 3, the gloss was enhanced to about 25%, and
a hot offset phenomenon did not occur and the higher gloss level was
maintained even when the fixing control temperature was as high as about
190° C.

[0077]The results of the third experiment are attributed to the fact that
the sheet P is warmed when passing through the fixing nip 17 in the first
heating and pressing process (preheating process) performed on the first
side thereof without forming a toner image. More specifically, when the
toner image is formed and fixed on the first side after the sheet P is
thus warmed, the toner image is warmed not only by the fixing roller 4,
but also by the sheet P, and thus the toner is melted more uniformly and
the surface of the toner image becomes smoother than in the first
experiment.

[0078]Further, it is possible that hot offset did not occur even when the
fixing control temperature was as high as about 190° C. because,
in the first experiment, a surface layer of the toner image is easily
removable because the toner differently melts in a portion close to the
fixing roller 4 and another portion close to the sheet P. By contrast,
when the preheating process is performed as in the experiment 3, the
toner becomes uniform throughout a toner layer immediately after passing
through the fixing nip 17, and thus the surface layer of the toner is not
easily removable.

[0079]In an image forming apparatus that has a function to perform
double-side processes automatically, the sheet P can be preheated by
passing through the fixing nip 17 where the sheet P is heated and pressed
without forming a toner image on a first side of the sheet P. By forming
a toner image on a second side of the sheet subsequently to the
preheating process, a toner image having an enhanced gloss can be
produced with hot offset prevented or reduced by using such an image
forming apparatus.

[0080]Described below is another embodiment in which the gloss of the
toner image can be further enhanced when a heating temperature to heat
the sheet P is higher in the first heating and pressing process
(preheating process) than in the second heating and pressing process
(fixing process).

[0081]FIG. 4 is a graph illustrating a relation between a heating control
temperature of the fixer 1 and a gloss level of the toner image both when
the heating temperature is identical and different in the first heating
and pressing process and the second heating and pressing process. The
lines 1 and 2 show results of the single pass (first experiment) and the
double pass (second experiment), respectively, similarly to those shown
in FIG. 3. A line 4 shows results of an experiment 4 in which the
preheating process was performed similarly to the experiments 3, and the
heating temperature was identical in both the first heating and pressing
process (preheating process) and the second heating and pressing process
(fixing process). A line 5 shows results of an experiment 5 in which the
heating temperature in the first heating and pressing process (preheating
process) was higher by 20° C. than that in the second heating and
pressing process (fixing process).

[0082]As is clear from the lines 4 and 5, the gloss of the toner image was
higher by about 2% in the experiment 5, in which the heating temperature
in the first heating and pressing process was higher by 20° C.
than that in the second heating and pressing process, than in the
experiment 4. Further, when the first side of the sheet P is thus
preheated, hot offset does not occur because no toner images are formed
on the first side of the sheet P, and thus a greater amount of heat can
be given to the sheet P.

[0083]Described below is another embodiment in which the gloss level of
the toner image can be enhanced by changing a fixing linear speed at
which the sheet P passes through the fixing nip 17 shown in FIG. 2 in the
first and second heating and pressing process, similarly to changing the
heating temperature in the first and second heating and pressing process
in the embodiment described with reference to FIG. 4.

[0084]FIG. 5 is a graph illustrating a relation between a heating control
temperature of the fixer 1 and a gloss level of the toner image when the
fixing linear speed is identical and different in the first and second
heating and pressing process. The line 4 shows the results of the
experiment 4 in which the fixing linear speed is identical in both the
first and second heating and pressing process, and a line 6 shows results
of an experiment 6 in which the fixing linear speed in the first heating
and pressing process was 90 mm/s and was slower by 50% than that in the
second heating and pressing process. The graph shown in FIG. 5 also
includes the lines 1 and 2 shown in FIG. 3 as references.

[0085]As is clear from the lines 4 and 6, the gloss of the toner image is
enhanced by lowering the fixing linear speed in the first heating and
pressing process from the fixing linear speed in the second heating and
pressing process. It is to be noted that the linear speed to transport
the sheet P is slowed only for a time period during which the sheet P
passes through the fixing nip 17. It is preferable to transport the sheet
P along the sheet reverse path at a predetermined or given linear speed
so as to reduce a decrease in temperature of the sheet P after the sheet
P passes through the fixing nip 17.

[0086]It is to be noted that a Konica Minolta GM-60 gloss meter, with a
measurement angle of 60 degrees, was used in the experiments 1 through 6
shown in FIGS. 3 through 5.

[0087]In the first heating and pressing process in which a heating member
and a pressure member give heat to the sheet P, the sheet P is likely to
curl up if a surface temperature of the pressure member is lower than
that of the heating member, particularly under humid conditions. If the
sheet P curls, a transport error is likely to occur when the sheet P
passes through the fixer 1, the sheet reverse unit 31, or the sheet
transport unit 32. Such a transport error includes jamming, in which a
leading edge of the sheet P gets stuck at an entry of each unit. To
prevent curling of the sheet P, it is preferable to detect an absolute
temperature inside the image forming apparatus and to control the surface
temperatures of the heating member and the pressure member based on the
absolute temperature thus detected.

[0088]Referring to FIG. 2, the inventors of the present invention have
discovered that curling of the sheet P does not affect transport of the
sheet P or does not occur when a certain relation is satisfied among the
absolute humidity inside the image forming apparatus, and surface
temperatures of the fixing belt 2 and the pressure roller 5. The relation
may be expressed as:

Y-Z≦(-6.7×X)-150 Expression 1

[0089]wherein Y is the surface temperature of the fixing belt 2 (°
C.), Z is the surface temperature of the pressure roller 5 in (°
C.), and X is an absolute humidity (g/m3) detected by the humidity
sensor 16.

[0090]If the relation shown by the expression 1 is not satisfied, it is
preferable to idle the fixer 1 so as to adjust the surface temperatures
of the fixing belt 2 and the pressure roller 5 and suspend feeding of the
sheet P until this relation is satisfied.

[0091]Alternatively, when the image forming apparatus includes a relative
humidity detector element and a temperature detector element, the
absolute humidity to be used in the expression 1 may be calculated by
using the relative humidity detector element and the temperature detector
element.

[0092]Further, a thinner sheet, particularly a sheet having a basic weight
of 60 g/m2 or less, is more likely to curl up than is a standard
sheet having a basic weight of greater than 60 g/m2. Therefore, when
the sheet P is such a thinner sheet, an expression 2, shown below, that
adjusts the expression 1 with a coefficient is effective.

Y-Z≦{(-6.7×X)-150}×0.9 Expression 2

[0093]It is to be noted that the control of the surface temperatures of
the fixing belt 2 and the pressure roller 5 based on the expression 2 is
also effective for a sheet having a basic weight within a range of from
60 to 80 g/m2, depending on conditions under which the sheet is
stored.

[0094]By using the expression 2, curling of the thinner sheets can be
prevented or reduced, and thus transport error can be prevented.

[0095]It is to be noted that the image forming apparatus may include a
detector to determine whether or not the sheet P is a thinner sheet.
Alternatively, the image forming apparatus may adopt a method in which
the relation used to control the surface temperatures of the fixing belt
2 and the pressure roller 5 is changed to the relation shown by the
expression 2 by the user indicating, via an input from an operation
panel, etc., that the sheet P is the thinner sheet described above.

[0097]As described in the description of the glossy image single-side
printing mode (M3+M1 mode), the user selects the glossy image mode M3 so
as to set the image forming apparatus 20 shown in FIG. 1 to the glossy
image mode M3, and further selects the double-side printing mode M2. In
this case, a sheet having coated layers on both sides thereof is used as
the sheet P and is contained, for example, in the sheet cassette 24b.

[0098]While the sheet P is transported, the toner image detector 36 shown
in FIG. 1 detects that no toner images are formed thereon. When the
glossy image mode M3 is set and the toner image detector 36 determines
that no toner images are formed on the first side of the sheet P, the
image forming units 21C, 21Y, 21M, and 21BK do not perform the image
forming processes. The registration rollers 30 and the transport belt 22a
transport the sheet P to the fixer 1 where the fixing roller 4 and the
pressure roller 5 perform the first heating and pressing process
(preheating process). Then, the sheet P is further transported to the
sheet reverse unit 31 by the switch claw, not shown.

[0099]After passing the sheet reverse unit 31, the sheet P is reversed by
the transport roller 35 and transported to the image forming units 21C,
21Y, 21M, and 21BK so that the respective color toner images thereon are
transferred and superimposed one on another on the coated layer on the
first side of the sheet P. Then, the sheet P is transported to the fixer
1 where the fixing roller 4 and the pressure roller 5 perform the second
heating and pressing process (fixing process) to fix the toner image
formed on the coated layer. The sheet P is again reversed while
transported through the sheet reverse unit 31, the sheet transport unit
32, and the transport roller 35, and transported to the image forming
units 21C, 21Y, 21M, and 21BK with the second side thereof up. While the
sheet P is transported along the image forming units 21C, 21Y, 21M, and
21BK, a toner image is formed on the second side of the sheet P, and then
the fixer 1 performs a third heating and pressing process to fix the
toner image. After the toner image is fixed, the sheet P is discharged
from the discharge port 34.

[0100]In the glossy image double-side printing mode, because the toner
image is fixed in the second heating and pressing process after the sheet
P is preheated in the first heating and pressing process, an excellent
glossy toner image can be formed.

[0101]Referring to the block diagram shown in FIG. 6, described below is a
control mechanism included in the image forming apparatus 20 that has the
functions described above.

[0102]As shown in FIG. 6, the image forming apparatus 20 includes an image
mode selection member 40, a printing mode selection member 41, a
controller 42, an image forming unit driving member 43, and a switch claw
driving member 44. The image mode selection member 40 selects one of the
glossy image mode M3 and the standard mode M4, and the printing mode
selection member 41 selects one of the single-side printing mode M1 and
the double-side printing mode M2.

[0103]According to the selection made by the image mode selection part 40
and the printing mode selection part 41, the controller 42 controls the
image forming unit driving part 43 so as to start and stop image
formation on the sheet P, following the procedure of one of the four
modes described above. Further, the controller 42 controls the switch
claw driving part 44 so as to switch the sheet transport path to one of
the sheet reverse unit 31 and the discharge port 34, according to the
selection made by the image mode selection part 40 and the printing mode
selection part 41.

[0104]The image mode selection part 40 and the printing mode selection
part 41 allow the user to select the modes described above on a control
panel, not shown, provided on the image forming apparatus 20.

[0105]The control mechanism further includes a sheet selection part 45,
the first temperature sensor 8a to detect the heating temperature of the
fixing belt 2, the second temperature sensor 8b to detect the heating
temperature of the pressure roller 5, a fixing condition input part 46,
and a fixer driving part 47.

[0106]The sheet selection part 45 determines whether or not the basic
weight of the sheet P is greater than 60 g/m2. The fixing condition
input part 46 determines which of the relations expressed by the
expressions 1 and 2, described above, should be applied based on input
from the sheet selection part 45, the humidity sensor 16, and the first
temperature sensor 8a and the second temperature sensor 8b, and sets the
fixing conditions. The controller 42 controls the fixer driving part 47
based on input from the fixing condition input part 46.

[0107]Further, when the image mode selection part 40 selects the glossy
image mode and inputs that selection to the controller 42, the controller
42 sets the heating temperature and the fixing linear speed in the first
heating and pressing process (preheating process) and controls the fixer
driving part 47 so that the fixer 1 operates according to the sent fixing
conditions.

[0108]It is to be noted that the fixer is not limited to the belt type
fixer 1 shown in FIG. 2, and alternatively, other types of fixers may be
used.

[0109]FIG. 7 shows an example of a roller type fixer 1A according to
another embodiment of the present invention. As shown in FIG. 7, the
fixer 1A includes a heating roller 50, a pressure roller 51, a thermistor
8c, a separation claw 11, and a guide 12 that guides the sheet P. The
thermistor 8c is provided to face the heating roller 50 and serves as a
temperature detector to detect a surface temperature of the heating
roller 50.

[0110]The fixer 1A fuses and fixes the toner image on the sheet P with
heat from the heating roller 50, while the sheet P is held in a fixing
nip 52 formed between the heating roller 50 and the pressure roller 51
and transported in the sheet transport direction shown by arrow D.

[0111]The heating roller 50 includes a metal core including aluminum and
is covered with a nonconductive PFA
(tetrafluoroethylene-perfluoroalkylvinylether copolymer) layer having a
thickness of about 20μ so as to secure releasability from the toner. A
heater 53 is provided inside the heating roller 50, and a heating
capacity thereof is 1200 W when a voltage of 100 V is applied.

[0112]The pressure roller 51 includes a metal core 9 and an elastic layer
10 that covers the metal core 9, and a conductive PFA tube that covers
the elastic layer 10 and has a thickness within a range of from 30μ to
50μ. The elastic layer 10 is a heat-resistant, porous material, such
as silicone foam rubber. The pressure roller 51 is urged by an elastic
member, such as a spring, in a direction to press against the heating
roller 50.

[0113]The separation claw 11 is located downstream of the fixing nip 52 in
the sheet transport direction shown by arrow D, with a tip thereof
pressed against the outer surface of the heating roller 50. When the
sheet P adheres to the outer surface of the heating roller 50, the
separation claw 11 separates the sheet P from the heating roller 50 by
entering therebetween while the sheet P is transported to prevented the
sheet P from winding around the heating roller 50.

[0114]An advantage of the roller type fixer described above is that it is
more compact than a belt type fixer.

[0115]FIG. 8 shows an example of an electromagnetic induction fixer 1B
using a free nip belt method, according to another embodiment of the
present invention.

[0116]As shown in FIG. 8, the fixer 1B includes a fixing roller 59 and a
pressure roller 60. The fixing roller 59 includes a heating member 56
including an excitation coil unit 54 and a magnetic metal member 55, a
film interior guide 57 to which the heating member 56 is attached, and a
cylindrical heat-resistant film 58. The film 58 covers the film interior
guide 57 with the magnetic metal member 55 contacting an inner wall
thereof. The pressure roller 60 contacts the film 58 where the magnetic
metal member 55 is located so as to form a fixing nip portion N with the
film 58, and rotates the film 58 as the free nip belt.

[0117]Examples of the film 58 include a heat-resistant single layer film
of PTFE (polytetra fluoro ethylene), PFA, FEP
(tetrafluoroethylene-hexafluoropropylene copolymer), etc., having a
thickness of not greater than 100μ, more preferably, within a range
from 20μ to 50μ. Alternatively, the film 58 may be a multilayer
film including a film of polyimide, polyamide-imide, PEEK (polyether
ether ketone), PES (polyether sulfone), PPS (polyphenylene sulfide), etc.
whose outer surface is coated with PTFE, PFA, FEP, etc.

[0118]The film interior guide 57 is a rigid, heat-resistant member
including resin such as PEEK, PPS, etc. The heating member 56 is attached
to a substantially central portion of the film interior guide 57 in a
longitudinal direction thereof.

[0119]The pressure roller 60 includes a core 60a and a heat-resistant
rubber layer 60b, such as silicone rubber, that covers the core 60a and
has better releasability. The pressure roller 60 is urged by a
predetermined or given bearing and/or an urging member, not shown, so as
to press against the magnetic metal member 55 of the heating member 56
via the film 58 with a predetermined or given pressure.

[0120]The pressure roller 60 is rotatively driven counterclockwise by a
driving member, not shown, and this rotation generates a frictional force
between the pressure roller 60 and the film 58. The rotation force acts
on the film 58, and the film 58 rotates slidably while contacting the
magnetic metal member 55 of the heating member 56. When the temperature
of the heating member 56 reaches a predetermined or given temperature,
the sheet P having unfixed toner images T formed by the image forming
units 21C, 21Y, 21M, and 21BK shown in FIG. 1 is transported between the
film 58 and the pressure roller 60 in the fixing nip portion N, in the
sheet transport direction shown by arrow D. While the sheet P is
sandwiched between the film 58 and the pressure roller 60 and transported
through the fixing nip portion N, the magnetic metal member 55 heats the
sheet P through the film 58, and thus the toner images T are fused and
fixed thereon.

[0121]At an exit of the fixing nip portion N, the sheet P is separated
from a surface of the film 58 and discharged onto a discharge tray, not
shown.

[0122]Such a free nip belt fixer described above responds quickly when a
control temperature is changed, and is suitable for the image forming
apparatus according to the present invention.

[0123]In addition, because the electromagnetic induction fixer 1B
described above uses eddy current, the magnetic metal member 55 that is a
heating member can be located close to the toner images T on the sheet P
via the film 58, with better heating efficiency than that of a fixer
using a film heating method.

[0124]Further, a fixer used in a full color image forming apparatus
requires a capacity to thoroughly fuse a thicker toner image having more
than four toner particle layers. Therefore, to meet this requirement, an
elastic rubber layer having a thickness of about 200 μm is formed on
the film 58 in the fixer 1B so that the toner image is overlapped fully
and fused uniformly.

[0125]The electromagnetic induction heater requires a shorter time period
to change the control temperature compared with a fixer using a halogen
heater, and thus it is suitable for the image forming apparatus according
to the present invention.

[0126]Described below is a suitable toner for the embodiments of the
present invention so as to produce excellent glossy images.

[0127]FIG. 9 is a graph showing stress change of the toner when the toner
is heated as data of the toner suitable for the present invention. The
graph shown in FIG. 9 shows that the toner starts to flow when heated to
over a predetermined or given temperature, which is herein after referred
to as a flowing start temperature (Tfb) and used as a melting start
temperature. The toner used in the present embodiment can produce high
quality images with enhanced transparency and saturation (brightness and
gloss) and excels in powder fluidity, hot-offset resistivity, charge
stability, and transfer properties, compared with a known pulverized
toner.

[0128]However, the toner used in the present embodiment starts to melt at
a relatively high temperature. More specifically, the toner used in the
present embodiment has a flowing start temperature of 92±1° C.,
while a known pulverized toner has a flowing start temperature of
86±2° C. Therefore, preheating the first side of the sheet P is
effective to fuse throughout a toner layer more uniformly so as to
improve smoothness of a toner surface and maintain a high level of gloss.

[0129]The flowing start temperature of the toner can be measured by using
a flow tester. An example of the flow tester is a Shimazu Corporation
CFT-500D flow tester. A flow curve shown in FIG. 9 is obtained by using
this flow meter, and various temperatures can be read therefrom. In the
present embodiment, measurements were performed under conditions of a
load of 5 kg/cm2, a heating rate of 3.0° C./min, a die bore
diameter of 1.00 mm, and a die length of 10.0 mm.

[0130]The toner used in the present embodiment is produced as follows:

[0131]A prepolymer including a modified polyester resin, a compound with
which the prepolymer is capable of elongating or cross-linking, and toner
components are dissolved or dispersed in an organic solvent, and then
elongating or cross-linking of the dissolutions or dispersions is allowed
in an aqueous medium. The toner is obtained by removing the solvent from
the dispersion.

[0132]It is to be noted that "and/or" means "at least one of".

[0133]The toner used in the present embodiment includes a polyester resin
as a binder, and a pigment as a colorant that disperses highly, and thus
the toner can produce high quality images that excel in transparency and
saturation (brightness and gloss). Further, the toner excels in powder
fluidity, resistance to hot offset, charge stability, and transfer
properties.

[0134]Features of the toner are described below.

[0135]1. To produce the toner, a prepolymer including a modified polyester
resin, a compound with which the prepolymer elongates or cross-links, and
toner components are dissolved or dispersed in an organic solvent, and
thus an oil-base dissolutions or dispersion liquid is obtained. The
dissolution or dispersion is allowed to elongate or cross-link in an
aqueous medium, and the toner is obtained by removing the solvent from
the dissolution or dispersion. The pigment (colorant) dispersed in the
toner particles has a dispersion particle diameter of 0.5 μm or
smaller by number average particle diameter, and a percentage of the
particles having a particle diameter of 0.7 μm or greater is not
greater than 5% by number.

[0136]Features 2 through 8 described below are added to the feature 1
described above independently or in combination.

[0137]2. The dispersion particle diameter of the pigment is 0.3 μm or
smaller, and the percentage of the particles having a particle diameter
of 0.5 μm or greater is not greater than 10% by number.

[0138]3. The toner particles have a weight average particle diameter
within a range of from 3.0 to 7.0 μm and a particle distribution of

1.00≦Dv/Dn≦1.20

[0139]wherein Dv is weight average particle diameter, and Dn is number
average particle diameter.

[0140]4. Toner particles have an average circularity within a range of
from 0.900 to 0.960.

[0141]5. The polyester resin included in the toner includes an element
that is soluble in tetrahydrofuran (tetrahydrofuran soluble element) and
has a main peak of a molecular weight distribution within a range from
2500 to 10,000, and a number average molecular weight of the
tetrahydrofuran soluble element is within a range from 2,500 to 50,000.

[0142]6. The polyester resin has a glass-transition temperature (Tag)
within a range of from 40° C. to 65° C. and an acid number
within a range of from 1 to 30 KOH mg/g.

[0143]7. The oil-base dissolution or dispersion liquid includes a
polyester resin unreactive with amine.

[0144]8. The toner described above is mixed with carrier in the developer
used in the present embodiment, and can be used as both monochrome toner
and color toner.

[0145]A method to manufacture the toner is described in further detail
below.

[0146]At least a polyester prepolymer A containing an isocyanate group is
dissolved, a pigment type colorant is dispersed, and a releasing agent is
dissolved or dispersed in an organic solvent, and thus an oil-base
dispersion liquid is formed. The dispersion liquid is dispersed in an
aqueous medium including an inorganic fine particles and/or polymer fine
particles. Further, in the dispersion liquid, the prepolymer A is allowed
to react with an amine B including a polyamine and/or a monoamine
containing an active hydrogen group so as to produce an urea-modified
polyester resin C. Then, the liquid medium is removed from the dispersion
liquid including the urea-modified polyester resin C, and thus the toner
is obtained.

[0147]It is to be noted that hereinafter a dispersed material referred to
as a material dissolved or dispersed in the oil-base dispersion liquid.

[0148]The urea-modified polyester resin C has a glass-transition
temperature within a range of from 40° C. to 60° C., and
this range is preferably from 45° C. to 60° C. Its number
average molecular weight Man is within a range from 2,500 to 50,000,
preferably within a range from 2,500 to 30,000, and its weight average
molecular weight Mw is within a range from 10,000 to 500,000, preferably
within a range from 30,000 to 100,000.

[0149]The toner includes, as a binder resin, the urea-modified polyester
resin C containing an urea binding whose molecular weight is increased by
the reaction between the prepolymer A and the amine B, and the colorant
is dispersed highly in the binder resin.

[0150]When the pigment type colorant contained in the toner particles is
controlled to have a dispersion particle diameter in number average of
0.5 μm or below, and the percentage of colorant particles having an
average particle diameter of not less than 0.7 μm is controlled 5% or
below by number, the toner excels in low temperature fixing properties,
charge stability, and fluidity. Further, the toner can produce high
quality images, particularly, color images having better transparency and
gloss.

[0151]Quality of the toner can be further enhanced by controlling the
pigment type colorant to have a dispersion particle diameter in number
average of not greater than 0.3 μm and the percentage of pigment
particles having an average particle diameter of not less than 0.5 μm
to 10% or below. Because such a toner excels in image resolution, the
toner is suitable for a digital developing device. In particular, color
toners excel in image resolution and transparency and produce images with
enhanced color reproducibility.

[0152]To obtain such a high quality toner in which the colorant is
dispersed uniformly, a process to pulverize the colorant
(wet-pulverization process) is required in forming the oil-base
dispersion liquid including the prepolymer A, the colorant, and the
releasing agent. The wet-pulverization process can be performed by any
wet-pulverization machine that can pulverize the colorant in a liquid
with an impact force. Examples of this wet-pulverization machine include
a ball mill and a bead mill. The wet-pulverization process is preferably
performed at a temperature within a range of from 5° C. to
20° C., and the range is more preferable from 15° C. to
20° C.

[0153]By adjusting conditions of the wet-pulverization process, the
colorant included in the toner can be controlled to have the dispersion
particle diameter and the particle diameter distribution described above.
It is to be noted that the wet-pulverization process is applicable to the
dispersion liquid after reaction, as required.

[0154]Further, to obtain the high quality toner described above, a method
using a master batch is applicable. More specifically, the master batch
is manufactured by dispersing the colorant in a resin in higher
concentration, and the master batch colorant particles are added to the
organic solvent as a colorant material and then dispersed therein by
agitation.

[0155]By using the master batch particles, the colorant particles having a
smaller dispersion particle diameter can be dispersed uniformly in the
toner, and the toner can produce high quality images with better
transparency. To manufacture preferable master batch particles, a mixture
including a thermally melting resin and the colorant is kneaded with a
higher shearing force at a melting temperature of this resin. The kneaded
material is cooled to solidify, and the solidified material is then
pulverized.

[0156]A preferable resin as the thermally melting resin is a thermoplastic
resin having a good miscibility with the urea-modified polyester resin C
that is produced by using the prepolymer A. The thermoplastic resin has a
softening point within a range of from 100° C. to 200° C.,
more preferable, from 120° C. to 160° C., and a number
average molecular weight Man within a range of from 2,500 to 50,000, more
preferable, from 2,500 to 30,000. The master batch has a colorant
concentration within a range of from 10% to 60% by weight, more
preferably, from 25% to 55% by weight.

[0157]Described below is a measurement method of toner properties
including the dispersion particle diameter of the colorant.

[0158]To measure the dispersion particle diameter and the particle
diameter distribution of the colorant included in the toner, a
measurement sample is prepared. To prepare the measurement sample, the
toner is embedded in an epoxy resin and sliced into flakes having a
thickness of about 100 nm by using a microtome MT 6000-Xl, manufacture by
RMC, Inc., for example.

[0159]Several transmission electron microphotographs (TEM photos) of these
measurement samples are taken by using an electron microscope, such as
H-9000NAR manufacture by Hitachi, Ltd., with an accelerating voltage of
100 kV and a magnification within a range of from 10,000 to 40,000. Image
information of the TEM photos is converted into image data with an image
analyzer, such as a NIRECO LUZEX III. Target colorant particles that are
particles having a particle diameter of not smaller than 0.1 μm are
measured randomly until its sampling number exceeds 300, and then its
average particle diameter and particle diameter distribution are
calculated.

[0160]The toner according to the present embodiment has a weight average
particle size (Dv) within a range of from 3 μm to 7 μm, and the
ratio of the weight average particle size ratio (Dv) to the number
average particle diameter (Dn) is 1.00≦Dv/Dn≦1.20. The
toner having higher resolution and image quality can be obtained by
specifying the ratio of the Dv to the Dn to this range.

[0161]To enhance image quality further, the percentage of the toner
particles having a particle diameter of not greater than 3 μm is
controlled to within a range from 1% to 10% by number, in addition to
maintaining the weight average particle diameter and the ratio of the Dv
to the Dn to the range described above. More preferably, the weight
average particle diameter is within a range from 3 μm to 6 μm, and
the ratio of the Dv to the Dn is 1.00≦Dv/Dn≦1.15. Such a
toner excels in thermal and storage stability, low temperature fixing
properties, and resistance to hot offset. Moreover, in the case of a
two-component developer, changes in the toner particle diameter is
relatively small even when consumption and supply of the toner repeated
for a relatively long time period, and good developability can be
maintained even when the toner is agitated in a developing device for a
relatively long time period.

[0162]Although the toner having a smaller particle diameter is generally
advantageous to obtain images with a higher resolution and image quality,
such a toner is disadvantageous to transfer and cleaning properties.
Further, when the toner has a weight average particle diameter of smaller
than the range specified in the present embodiment, the following
phenomena tend to occur: When such a toner is used in the two-component
developer and agitated for a relatively long time period in the
developing device, the toner is fused and deposits on a surface of the
carrier, and thus charging ability of the carrier impaired. By contrast,
when such a toner is used as one-component developer, the toner is likely
to adhere to the developing roller and/or a blade to regulate a thickness
of a toner layer on the developing roller.

[0163]The phenomena described above are closely related to a content rate
of fine particles in the toner. In particular, when the content rate of
fine particles having a particle diameter of not greater than 3 μm
exceeds 10%, the toner does not easily adheres to the carrier and it is
difficult to maintain a relatively high charge stability.

[0164]By contrast, when the toner has an average particle diameter of
greater than the range specified in the present embodiment, it is
difficult to obtain images with high resolution and image quality, and
changes in the toner particle diameter in the developer is likely to
increase while consumption and supply of the toner included in the
two-component developer are repeated. Further, this is also applicable
when the ratio of the Dv to the Dn is greater than 1.20.

[0165]The average particle diameter and the particle diameter distribution
of the toner are measured through the Coulter counter method. Examples of
an instrument to measure the particle distribution of the toner particles
include Coulter Counter TA-II and Coulter Multisizer II both manufactured
by Beckman Coulter, Inc. In the present embodiment, Coulter Counter TA-II
was used and connected to an interface to output number distribution and
volume distribution, manufactured by The Institute of Japanese Union of
Scientists and Engineers and a NEC PC9801 personal computer in the
measurement.

[0166]The measurement of the number distribution and the volume
distribution of the toner are described below. As a dispersant, 0.1 ml to
5 ml of a surfactant, preferably an alkylbenzene sulfonic acid, is added
to 100 ml to 150 ml of an electrolyte. It is to be noted that the
electrolyte refers to a sodium chloride solution having an elemental
sodium content of about 1% that is produced by using a first grade sodium
chloride, such as a Beckman Coulter ISOTON-II.

[0167]Further, 2 to 20 mg of the toner is added to the electrolyte as a
test sample, and then the electrolyte in which the tone is suspended is
dispersed with an ultrasonic disperser for a time period from 1 min to 3
min. The volume and the number of the toner particles are measured by
using the instrument described above with an aperture of 100 μm so as
to determine the weight distribution and the number distribution thereof.

[0169]The ratio of the Dv to the Dn was calculated based on the weight
average particle diameter (Dv) obtained from the volume distribution and
the number average particle diameter (Dn) obtained from the number
distribution.

[0170]To improve the resistance to hot offset, various methods have been
proposed including controlling the molecular weight distribution of the
binder resin. Methods to balance low temperature fixing properties and
resistance to hot offset, which contradict each other, includes the
following methods. One method uses a binder resin having a broader
molecular weight distribution. Another method uses a mixture resin that
has at least two molecular weight peaks and includes a component having a
higher molecular weight of within range from several hundred thousands to
several millions and another component having a lower molecular weight
within a range of from several thousands to several tens of thousands.

[0171]When a component of a higher molecular weight has a cross-linking
structure or in a state of gel, it is more effective to prevent hot
offset. However, for a full color toner, which requires high gloss and
transparency, using a large amount of such a higher molecular weight
component is not preferable. In the present embodiment, the toner can
have a higher resistance to hot offset as well as higher gloss and
transparency, by including urea-modified polyester resin having an
area-connection whose molecular weight is relatively high.

[0172]In the present embodiment, the molecular weight distribution of the
binder resin is measured by using GPC (gel permeation chromatography) as
described below.

[0173]A column is stabilized in a heat chamber at a temperature of
40° C. As a column solvent to be used under this temperature, THF
(Tetrahydrofuran) is put into the column at a speed of 1 ml/min. A THF
sample solution of the resin is adjusted to have a concentration within a
range of from 0.05% to 0.6% by weight, 50 μl to 200 μl of the THF
solution is added in the column. Prior to the measurement, a calibration
curve is prepared using several standard polystyrene samples having a
single distribution peak. The molecular weight distribution of the sample
is determined based on a relation between logarithmic values of the
calibration curve and a count number. As the standard polystyrene samples
that can be used to create the calibration curve, for example, the
samples having a molecular weight of 6×102,
2.1×103, 4×102, 1.75×104,
5.1×104, 1.1×105, 3.9×105,
8.6×105, 2×106 and 4.48×106 from
Pressure Chemical Co. or Tosoh Corporation are used. It is preferable to
use at least 10 standard polystyrene samples. As the detector, an RI
(refraction index) detector is used.

[0174]The binder component included in the toner typically has a molecular
weight main peak of within a distribution range from 2,500 to 10,000,
preferably within a range from 2,500 to 8,000, and more preferably within
a range from 2,500 to 6,000.

[0175]If a component having a molecular weight of less than 1,000
increases in volume, thermal and storage stability tends to degrade. By
contrast, if a component having a molecular weight of greater than 30,000
increases in volume, although low temperature fixing properties tend to
degrade, this degrading can be limited by controlling balance.

[0176]The content of the component having a molecular weight of greater
than 30,000 is within a range from 1% to 10%, and this content is
preferably within a range from 3% to 6% although this depends on toner
materials. If this content is less than 1%, resistance to hot offset is
insufficient, and if this content exceeds 10%, gloss and transparency are
impaired. The binder resin included in the toner has a number average
molecular weight Mn within a range of from 2,500 to 50,000, and a ratio
of its weight average molecular weight Mw to the number average molecular
weight Mn is not greater than 10. When this ratio (Mw/Mn) is greater than
10, its sharp melting property is insufficient and gloss is impaired.

[0177]The circularity of the toner is measured by a flow-type particle
image analyzer FPIA-2000 from SYSMEX CORPORATION, for example. The toner
according to the present embodiment has an average circularity within a
range of from 0.900 to 0.960. It is important that the toner has a
specific shape and a specific shape distribution.

[0178]If the average circularity is less than 0.900, because the toner
particles are amorphous and have insufficient transfer properties, it is
difficult to obtain high quality images without toner scattering. Such
amorphous toner particles have many contact points with a smooth member
such as the photoreceptor. Further, because electric charge tends to
concentrate on projection tips, such toner particles have higher Van der
Waals' forces and a higher mirror image force compared with relatively
spherical toner particles. Therefore, if the toner includes both
spherical particles and amorphous particles, the toner is partially
absent in a character portion and/or a line portion of a resulting image
because spherical particles are selectively transferred in the
electrostatic transfer process. Further, a cleaner is required to remove
the remaining toner that is not transferred, and toner yield, which is a
ratio of the toner used to form an image to total, is relatively low.

[0179]In the present embodiment, the circularity of the pulverized toner
is typically within a range from 0.910 to 0.920 when measured by the
analyzer described above.

[0180]Measurement of the toner shape (circularity) is described below.

[0181]A suitable method is an optical detection method in which a
suspension liquid including particles is passed through a plate-shaped
imaging detector, and an image of the particles is optically detected
with a CCD (charge-coupled device) camera. From this method, projection
area of the particle is available. The circularity is a value obtained by
dividing a circumference of a circle having an area identical with this
projection area by a circumference of an actual particle. This value is
the average circularity measured by the flow-type particle image analyzer
FPIA-2000 as described above.

[0182]The method is described below in details.

[0183]In a container, 100 ml to 150 ml of water from which impure solid
materials are previously removed is put, and 0.1 ml to 0.5 ml of a
surfactant as a dispersant, preferably an alkylbenzene sulfonic acid, and
further 0.1 g to 0.5 g of the toner (sample) are added thereto. The
suspension liquid including the sample is dispersed with an ultrasonic
disperser for 1 min to 3 min, and then a dispersion liquid having a
concentration within a range of from 3,000 to 10,000 pieces/μl is
prepared. The shape and shape distribution of the toner in the dispersion
liquid is measured with the instruments described above.

[0184]As described above, the toner manufacturing method according to the
present embodiment includes a molecular weight increasing process. In the
process, the polyester prepolymer A containing an isocyanate group is
dispersed in an aqueous medium including an inorganic fine particles
and/or polymer fine particles, and allowed to react with the amine B.

[0185]To manufacture the polyester prepolymer A containing an isocyanate
group, a polyester having an active hydrogen group that is formed through
polycondensation between polyol (PO) and polycarboxylic acid (PC) is
further reacted with polyisocyanate (PIC).

[0186]Examples of the active hydrogen contained in the polyester include a
hydroxyl group (an alcoholic hydroxyl group and a phenolic hydroxyl
group), an amino group, a carboxyl group, and a mercapto group. In
particular, the alcoholic hydroxyl group is preferably used.

[0187]As polyol (PO), diol (DIO) and a polyol having 3 or more valences
(TO) can be used. As a preferable polyol, the diol alone or a mixture of
the diol and a small amount of the TO is used.

[0188]Examples of diol include alkylene glycol such as ethylene glycol,
1,2-propylene glycol; bisphenol such as bisphenol A, bisphenol F and
bisphenol S; and adducts of the bisphenol with an alkylene oxide such as
ethylene oxide, propylene oxide and butylene oxide. In particular,
alkylene glycol having 2 to 12 carbon atoms and adducts of bisphenol with
an alkylene oxide are preferably used, and a mixture thereof is more
preferably used.

[0189]Examples of the TO having 3 or more valences include multivalent
aliphatic alcohol having 3 to 8 valences or more valences such as
glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and
sorbitol; phenol having 3 or more valences such as trisphenol PA,
phenolnovolak, cresolnovolak; and adducts of the above-mentioned
polyphenol having 3 or more valences with an alkylene oxide.

[0190]As the polycarboxylic acid (PC), dicarboxylic acid (DIC) and
polycarboxylic acid having 3 or more valences (TC) can be used, and DIC
alone, or a mixture of DIC and a small amount of TC are preferably used.
Examples of DIC include alkylene dicarboxylic acids such as succinic
acid, adipic acid and sebacic acid; alkenylene dicarboxylic acid such as
maleic acid and fumaric acid; and aromatic dicarboxylic acids such as
phthalic acid, isophthalic acid, terephthalic acid and naphthalene
dicarboxylic acid. In particular, alkenylene dicarboxylic acid having 4
to 20 carbon atoms and aromatic dicarboxylic acid having 8 to 20 carbon
atoms are preferably used. Examples of TC include aromatic polycarboxylic
acids having 9 to 20 carbon atoms such as trimellitic acid and
pyromellitic acid. Polycarboxylic acid (PC) can be formed from a reaction
between polyol (PO) and the above-mentioned acids anhydride or lower
alkyl ester such as methyl ester, ethyl ester and isopropyl ester.

[0191]As a ratio between polyol (PO) and polycarboxylic acid (PC), an
equivalent ratio [OH]/[COOH] between a hydroxyl group [OH] and a
carboxylic group [COOH] is typically within a range from 2/1 to 1/1,
preferably within a range from 1.5/1 to 1/1, and more preferably within a
range from 1.3/1 to 1.02/1.

[0192]Examples of polyisocyanate (PIC) include aliphatic polyisocyanate
such as tetramethylenediisocyanate, hexamethylene diisocyanate and
2,6-diisocyanate methylcaproate; alicyclic polyisocyanate such as
isophorone diisocyanate and cyclohexylmethane diisocyanate; aromatic
diisocyanate such as tolylene isocyanate and diphenylmethane
diisocyanate; aroma aliphatic diisocyanate such as α, α,
α', α'-tetramethylxylylene diisocyanate; isocyanurate; the
above-mentioned polyisocyanate blocked with phenol derivatives, oxime and
caprolactam; and their combinations.

[0193]As a ratio between polyisocyanate (PIC) and polyester (PE) resin in
manufacturing the polyester prepolymer A, an equivalent ratio [NCO]/[OH]
between an isocyanate group [NCO] and the polyester having a hydroxyl
group [OH] is typically within a range from 5/1 to 1/1, preferably within
a range from 4/1 to 1.2/1, and more preferably within a range from 2.5/1
to 1.5/1.

[0194]When the equivalent ratio [NCO]/[OH] is greater than 5, low
temperature fixing properties of the resultant toner are impaired. When a
molar ratio of the isocyanate group [NCO] is less than 1, a urea content
in ester of the modified polyester decreases and resistance to hot offset
of the toner is impaired.

[0195]The content of a polyisocyanate component in the polyester
prepolymer A that contains a isocyanate group at its end portion is
within a range from 0.5% to 40% by weight, preferably within a range from
1% to 30% by weight, and more preferably within a range from 2% to 20% by
weight. When the content is less than 0.5% by weight, in the resultant
toner, resistance to hot offset as well as the thermal and storage
stability and low temperature fixing properties are impaired. By
contrast, when the content is greater than 40% by weight, low temperature
fixing properties of the toner are impaired.

[0196]In one molecule of the polyester prepolymer A, typically more than
one isocyanate group is included. The number of this isocyanate group is
preferably from 1.5 to 3 on average, and more preferably from 1.8 to 2.5
on average. When less than one isocyanate group is included in one
molecule of the polyester prepolymer A, the molecular weight of the
urea-modified polyester decreases and resistance to hot offset of the
toner is impaired.

[0197]As the amine B, a polyamine and/or a monoamine containing an active
hydrogen group is used as described above. The active hydrogen group
includes a hydroxyl group and/or a mercapto group. Examples of the amine
B include diamines B1 such as aromatic diamines (e.g., phenylene diamine,
diethyltoluene diamine, and 4,4'-diaminodiphenyl methane); and polyamines
(B2) having three or more amino groups such as diethylene triamine and
triethylene tetramine. Among these compounds, the diamines (B1) and
mixtures of a diamine and a small amount of a polyamine (B2) are
preferably used.

[0198]In the reaction between the prepolymer A and the amine B, the
molecular weight of the polyester can be controlled by using an
elongation anticatalyst, as required. Examples of the elongation
anticatalyst include monoamines having no active hydrogen groups such as
diethyle amine, dibutyl amine, butyl amine and lauryl amine; and
compounds prepared by blocking these monoamines, such as ketimine
compounds. An amount of the anticatalyst to be used in this reaction is
determined in relation to a preferable molecular weight of the resultant
urea-modified polyester.

[0199]A ratio between the prepolymer A and the amine B is designated by an
equivalent ratio of the isocyanate groups [NCO] included in the
prepolymer A to the amino groups [NHx] included in the amine B. This
ratio [NCO]/[NHx] is typically within a range from 1/2 to 2/1, preferably
within a range from 1.5/1 to 1/1.5, and more preferably within a range
from 1.2/1 to 1/1.2. When the ratio [NCO]/[NHx] is greater than 2 or less
than 1/2, molecular weight of the polyester decreases, resulting in
deterioration of resistance to hot offset.

[0200]In the toner manufacturing method according to the present
embodiment, when the prepolymer A containing an isocyanate group is
reacted with the amine B in an aqueous medium, a polyester resin D that
is unreactive with amine can be used, as required. The polyester resin D
has a glass-transition temperature within a range of from 35° C.
to 65° C., preferably within a range from 45° C. to
60° C., and a number average molecular weight within a range of
from 2,000 to 10,000, preferably within a range from 2,500 to 8,000.

[0201]As the polyester resin D, a urea-modified polyester (UMPE) can be
used, and the urea-modified polyester may include an urethane bonding in
addition to a urea bonding.

[0202]A molar ratio of the urea bonding component to the urethane bonding
component is within a range from 100/0 to 10/90, preferably within a
range from 80/20 to 20/80, and more preferably within a range from 60/40
to 30/70. When the molar ratio of the urea bonding component is less than
10%, resistance to hot offset of the toner is impaired.

[0203]The urea-modified polyester can be produced by a known method such
as a one-shot method. The urea-modified polyester has a weight average
molecular weight of not less than 10,000, preferably within a range from
20,000 to 500,000, and more preferably within a range from 30,000 to
100,000. When the weight average molecular weight is less than 10,000,
resistance to hot offset is impaired.

[0204]In the toner manufacturing method according to the present
embodiment, the urea-modified polyester (UMPE) that is used as required
(polyester resin D) can be used alone, and alternatively, an unmodified
polyester resin (PE) can be included as a toner binder component in
combination with the urea-modified polyester (UMPE). Using the UMPE and
the PE in combination is preferable because low temperature fixing
properties of the toner and gloss of color images produced therewith can
be improved.

[0205]Examples of the unmodified polyester resin (PE) include
polycondensation products of polyol (PO) and polycarboxylic acid (PC)
that are similar to the polyester component of the UMPE. The unmodified
polyester (PE) has a preferable weight average particle diameter Mw that
is similar to that of urea-modified polyester resin (UMPE), as described
above. Alternatively, instead of the unmodified polyester resin,
polyester resins modified by a chemical bonding except for urea bonding,
such as urethane bonding, can be used with UMPE.

[0206]It is preferable that the unmodified polyester and the urea-modified
polyester are partially soluble with each other to improve low
temperature fixing properties and resistance to hot offset of the
resultant toner. Therefore, the unmodified polyester and the
urea-modified polyester preferably have similar structures. When the
unmodified polyester is used, a weight ratio of the urea-modified
polyester to the unmodified polyester (UMPE/PE) is typically within a
range from 5/95 to 80/20, preferably within a range from 5/95 to 30/70,
and more preferably within a range from 5/95 to 25/75. It is further
preferable when this ratio is within a range from 7/93 to 20/80. When the
weight ratio of the urea-modified polyester resin is less than 5%, the
resultant toner has poor resistance to hot offset, and it is
disadvantageous to balance thermal and storage stability and low
temperature fixing properties.

[0207]The unmodified polyester resin (PE) preferably has a hydroxyl value
not less than 5 mg KOH/g and an acid value within a range of from 1 mg
KOH/g to 30 mg KOH/g, more preferably within a range from 5 mg KOH/g to
20 mg KOH/g. The PE having such an acid value easily becomes negatively
charged, and the resultant toner has good affinity with a paper and low
temperature fixing properties are improved. However, when the acid value
is greater than 30 mg KOH/g, chargeability of the resultant toner is
impaired particularly due to changes in environmental conditions. In a
polyaddition reaction, fluctuation of the acid value causes a crush of
particles in a granulation process and it is difficult to control
emulsification.

[0208]In the present embodiment, the toner binder has a glass-transition
temperature within a range of from 45° C. to 60° C., more
preferably within a range from 45° C. to 60° C. Heat
resistivity is impaired when this glass-transition temperature is lower
than 45° C., and low temperature fixing properties are impaired
when this glass-transition temperature exceeds 65° C.

[0209]Examples of the colorant for use in the toner manufacturing method
according to the present embodiment include various known dyes and
pigments such as carbon black, Nigrosine dyes, anthraquinone green,
titanium oxide, zinc oxide, lithopone, and mixtures of these. The content
of the colorant in the toner is typically within a range from 1% to 15%
by weight, preferably within a range from 3% to 10% by weight.

[0210]As described above, it is preferable to use the master batch
colorant particles in which the colorant is combined with the resin.
Examples of the binder resin to be used with the colorant in
manufacturing the master batch include the modified and unmodified
polyester resins described above, polystyrene, terpene resins, aliphatic
or alicyclic hydrocarbon resins, and aromatic petroleum resins. These
resins can be used alone or in combination.

[0211]The master batch is typically prepared by mixing and kneading the
resin and the colorant with high shear stress. In this time, an organic
solvent can be used to enhance the interaction of the colorant with the
resin. Alternatively, flushing methods can be used in which an aqueous
paste including a colorant is mixed with a resin solution of an organic
solvent so as to transfer the colorant to the resin solution, and then
the aqueous liquid and organic solvent are separated and removed
therefrom. These methods are preferable because the resultant wet cake of
the colorant can be used as it is. To knead and mix the resin and the
colorant, a three-roll mill applying high shear stress is preferably
used.

[0212]In the toner, a releasing agent is included as wax together with the
toner binder and the colorant, and various known releasing agents can be
used. Examples of the releasing agent include polyolefin, long-chain
hydrocarbon, and waxes including a carbonyl group. Among these, the
carbonyl group containing waxes are preferable, and examples thereof
include polyalkanoic acid ester, polyalkanol ester, and dialkyl ketone
such as distearyl ketone. Among these carbonyl group containing waxes,
polyalkanoic acid ester is preferable.

[0213]The wax typically has a melting point within a range of from
40° C. to 160° C. This range is preferably within a range
from 50° C. to 120° C., and more preferably within a range
from 60° C. to 90° C. The wax adversely affects thermal and
storage stability if its melting point is lower than 40° C., and
the wax is likely to cause cold offset in fixing at a lower temperature
if its melting point is higher than 160° C.

[0214]Further, the wax preferably has a melt viscosity within a range of
from 5 cps (centipoise) to 1,000 cps, more preferably within a range from
10 cps to 100 cps, as a measurement value at a temperature higher by
20° C. than its melting point. If the wax has a melt viscosity
greater than 1,000 cps, the wax does not contribute much to the
improvement of resistance to hot offset and low temperature fixing
properties. The content of the wax in the toner is typically within a
range from 0 to 40% by weight, and this range is preferably from 3% to
30% by weight.

[0215]The toner used in the present embodiment may optionally include a
charge controller. Examples of the charge controller include various
known charge controllers such as Nigrosine dyes, triphenylmethane dyes,
quinacridone, azo pigments, and polymers having a functional group such
as a sulfonate group, a carboxyl group, a quaternary ammonium group, etc.

[0216]The content of the charge controller depends on the type of the
binder resin, whether or not an additive is added, and the toner
manufacturing methods (such as dispersion method), and is not
particularly limited. Typically, the content of the charge controller is
within a range from 0.1 to 10 parts by weight, and preferably within a
range from 0.2 to 5 parts by weight, to 100 parts by weight of the binder
resin.

[0217]If the content is greater than 10 parts by weight, the toner has
excessive chargeability, and thus main effects of the charge controller
is impaired. Further, the electrostatic force of a developing roller
attracting the toner increases, and thus fluidity of the toner and image
density are decreased. These charge controller and releasing agent can be
kneaded together with the master batch pigment and the resin.
Alternatively, the charge controller and the release agent can be added
when such toner constituents are dissolved or dispersed in the organic
solvent.

[0218]To improve the toner in fluidity, developability, and chargeability,
an additive can be externally added to the toner particles. Inorganic
particulate materials are preferable as the external additive. The
inorganic particulate material preferably has a primary particle diameter
within a range of from 5 nm to 2 μm, and more preferably within a
range from 5 nm to 500 nm. Further, the inorganic particulate material
preferably has a specific surface area within a range of from 20
m2/g to 500 m2/g determined through the BET (Brunauer, Emmett
and Teller) method.

[0219]An amount of the inorganic particulate material used in the toner is
preferably within a range from 0.01% to 5% by weight, more preferably
within a range from 0.01% to 2.0% by weight. Example of the inorganic
particulate material include silica and silicon nitride.

[0220]Alternatively, polymer particulate materials can be used instead of
inorganic particulate materials. Examples of inorganic particulate
materials include polystyrene formed by a soap-free emulsion
polymerization, a suspension polymerization, or a dispersion
polymerization; ester methacrylate; ester acrylate copolymer;
polycondensates such as silicone, benzoguanamine, and nylon; and
polymeric particulate materials formed of thermosetting resins.

[0221]For such an external additive, a surface treatment agent can be used
to increase the hydrophobic properties so as to prevent deterioration of
fluidity and chargeability even in an high humidity environment.
Preferable examples of the surface treatment agent include a silane
coupling agent, a sililation agent, a silane coupling agent having an
alkyl fluoride group, an organic titanate coupling agent, an aluminum
coupling agent, silicone oil, and a modified silicone oil.

[0222]A cleaning improver can be used to better remove the developer
remaining on the photoreceptor or a primary transfer medium after the
transfer process. Examples of the cleaning improver includes fatty acid
metallic salts such as zinc stearate, calcium stearate, and stearic acid;
and polymer particles prepared by, for example, a soap-free emulsifying
polymerization method, such as polymethyl methacrylate particles, and
polystyrene particles. The polymer particles relatively have a narrow
particle diameter distribution and preferably have a volume average
particle diameter within a range of from 0.01 μm to 1 μm.

[0223]Procedure of the toner manufacturing method according to the present
embodiment is described below.

[0224]Firstly, in an organic solvent, the isocyanate group containing
polyester prepolymer A is dissolved, the colorant is dispersed, and the
releasing agent is dissolved or dispersed, and thus the oil-base
dispersion liquid is prepared (oil-base dispersion liquid preparation
process). Then, in the wet-pulverization process, the oil-base dispersion
liquid is put in an wet-pulverization machine to pulverize and uniformly
disperse the colorant. The pulverization time is within a range from
about 30 min to 120 min.

[0225]Subsequently to the wet-pulverization process, a dispersion process
(emulsification process) and a reaction process are performed. More
specifically, in the dispersion process, the oil-base dispersion liquid
is dispersed or emulsified in an aqueous medium that includes inorganic
particulate and/or polymer particulates so as to produce an oil-in-water
dispersion liquid (emulsion liquid). Further, the isocyanate group
containing polyester prepolymer A is reacted with the amine B so as to
produce the urea-modified polyester resin C including an urea binding in
the reaction process.

[0226]The organic solvent to be used in an organic solvent is such an
organic solvent hat can dissolve a polyester resin therein and is
insoluble to water or has a lower solubility to water. The organic
solvent typically has a boiling point within a range of from 60°
C. to 150° C., and the boiling point is preferably within a range
from 70° C. to 120° C. Examples of the organic solvent
include ethyl acetate and methylethyl ketone.

[0227]As described above, it is preferable to use the master batch
colorant particles as the colorant so as to efficiently disperse the
colorant uniformly.

[0228]In the present embodiment, it is preferable to dissolve the
polyester resin D that is unreactive with amine in the organic solvent,
as an adjuvant. Further, the polyester resin D may be dispersed in an
aqueous medium.

[0229]A dispersion machine to dispersed the oil-base dispersion liquid in
an aqueous medium is not particularly limited, and the examples of the
dispersion machine includes known mixers and known dispersion machines
such as low shearing force type dispersing machines, high shearing force
type dispersing machines, friction type dispersing machines, high
pressure jet type dispersing machines, and ultrasonic dispersing machine.
In order to prepare particles having an average particle diameter within
a range of from 2 μm to 20 μm, high shearing force type dispersion
machines are preferably used. When high shearing force type dispersion
machines are used, the rotation speed is generally within a range from
1,000 rpm to 30,000 rpm and preferably within a range from 5,000 rpm to
20,000 rpm, but is not limited thereto. Further, the dispersion time is
generally within a range from 0.1 min to 5 min for a batch type machine,
but is not limited thereto. The dispersion process is typically performed
at a temperature within a range of from 0 to 150° C. (under
pressure), and this range is preferably within a range from 40° C.
to 98° C. In order to easily disperse the oil-base dispersion
liquid in an aqueous medium, a higher temperature is preferable because
the viscosity thereof is lower.

[0230]The content of the aqueous medium to 100 parts by weight of the
toner solid components including the prepolymer A, the colorant, the
releasing agent, and the polyester resin D, is typically from 50 to 2,000
parts by weight, and preferably from 100 to 1,000 parts by weight. When
the content is less than 50 parts by weight, the toner solid components
are inadequately dispersed in the aqueous medium, and thus the particle
diameter of the resultant toner particles are inadequate. By contrast,
when the content is greater than 2,000, the production cost is not
practical. As required, a dispersant can be used. Using a dispersant is
preferable to have a sharp particle diameter distribution as well as
stable dispersion. It is preferable to immediately disperse the oil-base
dispersion liquid in the aqueous liquid after the wet-pulverization
process.

[0231]As the aqueous medium, water can be used alone, and alternatively, a
solvent miscible with water can be mixed with water. Examples of such a
water-miscible solvent include alcohols such as methanol, isopropanol and
ethylene glycol; dimethylformamide; tetrahydrofuran; cellosolves such as
methyl cellosolve; and lower ketones such as acetone, and methyl ethyl
ketone.

[0233]Further, in the case of a surfactant including a fluoroalkyl group,
even a relatively small amount of the surfactant is effective. Examples
of anionic surfactants having a fluoroalkyl group include fluoroalkyl
carboxylic acids having from 2 to 10 carbon atoms and their metal salts,
disodium perfluorooctanesulfonylglutamate, sodium
3-{omega-fluoroalkyl(C6-C11)oxy}-1-alkyl(C3-C4) sulfonate,
sodium-{omega-fluoroalkanoyl(C6-C8)-N-ethylamino}-1-propanesulfonate,
fluoroalkyl(C11-C20) carboxylic acids and their metal salts,
perfluoroalkylcarboxylic acids and their metal salts,
perfluoroalkyl(C4-C12)sulfonate and their metal salts,
perfluorooctanesulfonic acid diethanol amides,
N-propyl-N-(2-hydroxyethyl)perfluorooctanesulfone amide,
perfluoroalkyl(C6-C10)sulfoneamidepropyltrimethylammonium salts, salts of
perfluoroalkyl(C6-C10)-N-ethylsulfonyl glycin,
monoperfluoroalkyl(C6-C16)ethylphosphates, etc.

[0234]Example of commercially available surfactants having a fluoroalkyl
group include SURFLON S-111, S-112 and S-113, which are manufactured by
Asahi Glass Co., Ltd.; FRORARD FC-93, FC-95, FC-98 and FC-129, which are
manufactured by Sumitomo 3M Ltd.; UNIDYNE DS-101 and DS-102, which are
manufactured by Daikin Industries, Ltd.; MEGAFACE F-110, F-120, F-113,
F-191, F-812 and F-833 which are manufactured by Dainippon Ink and
Chemicals, Inc.; ECTOP EF-102, 103, 104, 105, 112, 123A, 306A, 501, 201
and 204, which are manufactured by Tohchem Products Co., Ltd.; FUTARGENT
F-100 and F150 manufactured by Neos; etc.

[0236]As the inorganic particulate to be included in the aqueous medium,
various known inorganic particulates that are insoluble to water or has a
relatively low water-solubility can be used. Examples of such inorganic
particulates include tricalcium phosphate, calcium carbonate, titanium
oxide, colloidal silica and hydroxyapatite.

[0237]As the polymer particulate, various known polymer particulates that
are insoluble to water or has a relatively low water-solubility can be
used. Examples of such polymer particulates include hydrophobic
macromolecular particulates such as carbon hydride resins,
fluorine-containing resins, and silicone resins.

[0238]The particulates described above typically have a particle diameter
smaller than that of the toner particles. For the uniformity of the
particle diameter, the ratio of the volume average particle diameter of
the particulates to that of the toner particles is preferably within a
range from 0.001 to 0.3. If this ratio is greater than 0.3, the
particulates are not efficiently absorbed on the surfaces of the toner
particles, and thus the toner tends to have a broader particle diameter
distribution.

[0239]The volume average particle diameter of the particulates can be
adjusted so as to obtain the toner having a preferable particle diameter,
as long the particle diameter ratio described above is satisfied. For
example, to obtain the toner having a volume average particle diameter of
5 μm, the volume average particle diameter of the particulates is
preferably within a range of from 0.0025 μm to 1.5 μm, and more
preferably within a range from 0.005 μm to 1.0 μm. By contrast, to
obtain the toner having a volume average particle diameter of 10 μm,
the volume average particle diameter of the particulates is preferably
within a range of from 0.005 μm to 3 μm, and more preferably within
a range from 0.05 μm to 2 μm.

[0240]Further, as a dispersion stabilizer, various hydrophilic polymeric
substances that form polymeric protection colloid therein may be included
in the aqueous medium. Examples of a monomer component of such polymeric
substances include acids such as acrylic acid and methacrylic acid; and
vinyl monomers having a nitrogen atom or a nitrogen heterocycle such as
vinyl imidazole and ethylene imine. Other polymeric substances that can
be preferably used in the present embodiment includes polyoxyethylene
compounds such as polyoxyethylene, polyoxypropylene, polyoxyethylenealkyl
amines, polyoxypropylenealkyl amines, polyoxyethylenealkyl amides,
polyoxypropylenealkyl amides, polyoxyethylene nonylphenyl ethers,
polyoxyethylene laurylphenyl ethers, polyoxyethylene stearylphenyl
esters, and polyoxyethylene nonylphenyl esters; and cellulose compounds
such as methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl
cellulose.

[0241]In a process to remove the liquid medium from the emulsion
dispersion obtained after the polyaddition reaction between the
prepolymer A and the amine B (liquid medium removal process), an
adoptable method includes a process in which an entire system is
gradually heated so as to evaporate and remove the organic solvent. The
circularity of the toner can be controlled by adjusting liquid agitation
before the organic solvent is removed and a removal time of the organic
solvent.

[0242]More specifically, when the organic solvent is slowly removed from
the emulsion dispersion, the toner particles is more spherical with a
circularity of greater than 0.980. By contrast, when the dispersion
liquid is strongly agitated and the organic solvent is removed from the
emulsion dispersion in a relatively short time period, the toner
particles have rough surfaces and are amorphous with a circularity within
a range of from 0.900 to 0.950.

[0243]Further, when the liquid medium is removed while the emulsion
dispersion, obtained after the dispersion and the reaction, is agitated
relatively strongly in an agitation tank at a temperature within a range
of from 30° C. to 50° C., the toner circularity can be
controlled within a range from 0.850 to 0.990. This is because the
organic solvent, such as ethyl acetate, etc., is quickly removed during
granulation, and thus the volume is contracted.

[0244]Alternatively, in another method adoptable in the liquid medium
removal process, while the emulsion dispersion is sprayed in a dry
atmosphere and the organic solvent is completely removed so as to form
the toner particles, the aqueous dispersant is evaporated and removed.
The atmosphere into which the emulsion dispersion is sprayed includes
gases obtained by heating air, nitrogen, carbon dioxide, and combustion
gas, and gases preferable used are various airflows that are heated to a
temperature higher than a highest boiling point among boiling points of
the liquid media used in the dispersion liquid. A high quality toner can
be manufactured through a short period process by a spray drier, a belt
drier, a rotary kiln, etc.

[0245]It is to be noted that the liquid medium removal process is
preferably performed immediately after the reaction of the dispersion
liquid, and is typically performed within 25 hours after the reaction.

[0246]When a substance that is soluble in acid or alkaline, such as
calcium phosphate, is used as inorganic particulates, such inorganic
particles can be dissolved in acid or alkaline so as to be removed from
the toner particles. For example, calcium phosphate is dissolved with an
acid such as a hydrochloric acid, and then the toner particles are washed
with water to remove the calcium phosphate therefrom. Alternatively,
inorganic particulates may be removed by an enzymatic hydrolysis.

[0247]When a dispersant is used, although the dispersant may be allowed to
remain on the surfaces of the toner particles, for chargeability of the
toner, such a dispersant is preferably washed off from the toner
particles after the reaction between the prepolymer A and the amine B.

[0248]Further, in order to reduce viscosity of the dispersion liquid after
the reaction, a solvent capable of dissolving the prepolymer A, the
area-modified polyester, etc., can be included in the aqueous medium.
Using such a solvent is preferable because the resultant particles have a
relatively sharp particle diameter distribution.

[0249]The solvent is preferably volatile and has a boiling point lower
than 100° C. so as to be removed easily. Examples of such a
solvent include toluene, xylene, benzene, carbon tetrachloride, methylene
chloride, and 1,2-dichloroethane, and these can be used alone or in
combination. In particular, aromatic solvents such as toluene and xylene;
and halogenated hydrocarbons such as methylene chloride,
1,2-dichloroethane, chloroform, and carbon tetrachloride are preferably
used. Usage of such a solvent is from 0 to 300 parts by weight,
preferably from 0 to 100, and more preferably from 25 to 70 parts by
weight, to 100 parts by weight of the prepolymer A. After the reaction
between the prepolymer A and the amine B, such a solvent is heated under
a normal or reduced pressure to be removed from the dispersion liquid.

[0250]A reaction time period between the prepolymer A and the amine B is
determined based on reactivity that depends on combination of the
prepolymer A and the amine B, and is typically within a range from 10
minutes to 40 hours and preferably within a range from 2 hours to 24
hours. A temperature under which the prepolymer A reacts with the amine B
(reaction temperature) is typically within a range from 0° C. to
150° C., and preferably within a range from 40° C. to
98° C. As required, a known catalyst such as dibutyltinlaurate and
dioctyltinlaurate can be used.

[0251]When the toner particles in the emulsion dispersion has a relatively
broad particle distribution after the reaction between the prepolymer A
and the amine B, and the toner particles are to be cleaned and dried with
such a particle distribution maintained, the toner particles can be
classified into a preferred distribution.

[0252]To classify the toner particles, a particulate portion is removed in
the liquid by using a cyclone separator, a decanter, a centrifugal
separation, etc. Although the toner particles may be classified after
being dried into powder, the classification can be performed more
effectively in the liquid. The removed unnecessary particulates and/or
rough particles can be reused in a kneading process to form toner
particles, regardless of its being wet or dry.

[0253]It is to be noted that the dispersant is preferably removed from the
dispersion liquid as much as possible, preferably while the
classification is performed.

[0254]When the dried toner particles are mixed with a different type of
particles, such as releasing agent particles, charging control particles,
and fluidizing particles, by giving a mechanical impact to the mixed
powder, the different type of particles can be fused on the surfaces of
the toner particles so as not to separate therefrom.

[0255]For example, such a mechanical impact may be given to the mixed
powder by a blade rotating at a high speed. Alternatively, the mixed
powder is put in a high-speed airstream and is crashed into a impinging
plate. Examples of machines to give a mechanical impact include a
Hosokawa Micron Ong mill, a Nippon Pneumatic mill that is modified to
have a reduced pulverization air pressure, a Nara Machinery hybridization
system, a Cryptron system manufactured by Kawasaki Heavy Industries Ltd.,
and automatic mortars.

[0256]When the toner according to the present embodiment is used in a
two-component developer, the toner is mixed with an magnetic carrier. A
ratio of the toner is preferably within a range from 1 to 10 parts by
weight to 100 parts by weight of the magnetic carrier. Examples of the
magnetic carrier includes iron powders, ferrite powders, magnetite
powders, magnetic resin carriers that have a particle diameter within a
range of from about 20 μm to 200 μm. A surface of the carrier may
be coated by a silicone resin, a fluorine-containing resin. Further, such
a resin coat can include a electroconductive powder, as required.
Examples of the electroconductive powder includes metal powder, carbon
black, titanium oxide, and zinc oxide. Such a electroconductive powder
preferably has an average particle diameter of not greater than 1 μm.
When its particle diameter is larger than 1 μm, it is hard to control
electrical resistance of the resultant toner.

[0257]Alternatively, the toner according to the present embodiment can be
used as a one-component developer or a nonmagnetic toner.

[0258]The toner according to the present embodiment is further described
below using examples. Hereinafter "part(s) refer to "part(s) by weight".
Characteristics of the toner of the examples are shown in FIG. 10.

EXAMPLE 1

[0259]In the example 1, a polyester to be added was manufactured as
follows: In a reaction tank provided with a cooling pipe, an agitator,
and a nitrogen introduction pipe, 690 parts of bisphenol A with 2 moles
of ethylene oxide and 230 parts of terephthalic acid were reacted with
each other for 10 hours at a temperature of 210° C., under normal
pressure, and thus polycondensed. The reaction was further continued for
5 hours under a reduced pressure within a range of from 10 mmHg to 15
mmHg, and cooled to a temperature of 160° C. The mixture was
further reacted for 2 hours with 18 parts of phthalic anhydride added
thereto, and thus unmodified polyester (a) having a weight average
molecular weight (Mw) of 85,000 was obtained.

[0260]In the example 1, a prepolymer was prepared as follows: In a
reaction tank provided with a cooling pipe, an agitator, and a nitrogen
introduction pipe, 800 parts of bisphenol A with 2 moles of ethylene
oxide and 160 parts of isophthalic acid, 60 parts of terephthalic acid,
and 2 parts of dibutyl tin oxide were reacted with each other for 8 hours
at a temperature of 230° C., under normal pressure, and were
further reacted for 5 hours while being dehydrated under a reduced
pressure within a range of from 10 mmHg to 15 mmHg. Then, the mixture was
cooled to a temperature of 160° C. and further reacted for 2 hours
with 32 parts of phthalic anhydride added thereto. The mixture was then
cooled to a temperature of 80° C. Further, the mixture was reacted
with 170 parts of isophorone diisocyanate in ethyl acetate for 2 hours,
and thus an isocyanate group containing prepolymer 1 having a weight
average molecular weight of 35,000 was obtained.

[0261]In the example 1, a ketimine compound was prepared as follows: In a
reaction tank provided with an agitation bar and a thermometer, 30 parts
of isophorone diamine and 70 parts of methylethyl ketone were reacted
with each other for 5 hours at a temperature of 50° C., and thus
ketimine compound 1 was produced.

[0262]In the example 1, a toner was prepared as follows: In a beaker, 14.3
parts of the prepolymer 1, 55 parts of the polyester (a), and 78.6 parts
of ethyl acetate were agitated and solved. Further, 10 parts of a rice
wax that was a releasing agent having a melting point of 83° C.,
and 4 parts of copper phthalocyanine blue pigment were added to the
mixture. Then, the mixture was agitated for 5 minutes at a temperature of
40° C. by a TK homomixer at a revolution of 12,000 rpm, and
pulverized by using a beads mill for 30 minutes at a temperature of
20° C., and thus a oil-base dispersion 1 was obtained.

[0263]In a beaker, 306 parts of ion-exchanged water, 265 parts of 10%
tricalcium phosphate suspension liquid, and 0.2 part of sodium
dodecylbenzenesulfonate were put. While the mixture was agitated by a TK
homomixer at a revolution of 12,000 rpm as an aqueous dispersion liquid
1, the oil-base dispersion 1 and 2.7 parts of the ketimine compound 1
were added thereto and urea-reaction was induced.

[0264]After the reaction, the organic solvent was removed from the
dispersion liquid having a viscosity of 3,500 mPs (milli-Pascal second)
at a temperature of not higher than 50° C. under a reduced
pressure within 1 hour. The dispersion liquid was then filtered, washed,
dried, and classified with air, and thus spherical mother toner particles
1 were obtained.

[0265]Further, 100 parts of the mother particles 1, 0.25 part of a charge
controller, BONTRON E-84 manufactured by Orient Chemical Industries Co.,
Ltd., were mixed together in a Q-type mixer manufactured by Mitsui Mining
Co., Ltd., with a peripheral speed of turbine blades set to 50 m/s. In
this process, a two-minutes mixing and a one-minute halt were repeated
for 5 cycles, and a total operation time was 10 minutes. Further, 0.5
part of hydrophobic silica, H2000 manufacture by Clariant Japan, was
mixed with the mixture for performing 5 cycles of 30-second mixing at a
peripheral speed of 15 m/s and a 1-minute haut.

[0266]Thus, a cyan toner 1 was obtained. Its average dispersion particle
diameter was 0.4 μm, and a ratio of particles having a diameter of 0.7
μm or greater was 3.5% by number.

EXAMPLE 2

[0267]A magenta master batch was prepared as follows: By using a flusher,
600 parts of water and 200 parts of a cake containing Pigment Red 57 with
solid component of 50% were thoroughly agitated. To the mixture, 1,200
parts of a polyester resin (acid value: 3, hydroxyl group value: 25, Mm:
3,500, Mw/Mn: 4.0, Tg: 60° C.) was added and kneaded for 30
minutes at a temperature of 150° C., and further kneaded for 1
hours with 1,000 parts of xylene added thereto. After water and xylene
were removed therefrom, the mixture was rolled and cooled, and then
pulverized by a pulverizer. The mixture was further pulverized by a
three-roll mill for 2 passes, and thus a magenta master batch, MB1-M,
having a average particle diameter of 0.2 μm was obtained.

[0268]In example 2, a prepolymer was prepared as follows: In a reaction
tank provided with a cooling pipe, an agitator, and a nitrogen
introduction pipe, 856 parts of bisphenol A with 2 moles of ethylene
oxide and 200 parts of isophthalic acid, 20 parts of terephthalic acid,
and 4 parts of dibutyl tin oxide were reacted with each other for 6 hours
at a temperature of 250° C., under normal pressure, and were
further reacted for 5 hours while being dehydrated under a reduced
pressure within a range of from 50 mmHg to 100 mmHg. Then, the mixture
was cooled to a temperature of 160° C. and further reacted for 2
hours with 18 parts of phthalic anhydride added thereto. The mixture was
then cooled to a temperature of 80° C., and further reacted with
170 parts of isophorone diisocyanate in ethyl acetate for 2 hours, and
thus an isocyanate group containing prepolymer 2 having a weight average
molecular weight (Mw) of 25,000 was obtained.

[0269]In the example 2, a toner was prepared as follows: In a beaker, 15.4
parts of the prepolymer 2, 50 parts of the polyester (a), and 95.2 parts
of ethyl acetate were agitated and solved. Further, 10 parts of carnauba
wax having a molecular weight of 1800, an acid vale of 2.5, and a needle
penetration at 40° C. of 1.5 mm, and 10 parts of magenta master
batch particles MB1-M were added to the mixture, and then the mixture was
agitated at a temperature of 85° C. by a TK homomixer at a
revolution of 10,000 rpm. Further, the mixture was pulverized by using a
beads mill in a wet-pulverization process similar to the process
performed in the example 1, and thus a oil-base dispersion 2 was
obtained.

[0270]An aqueous dispersion liquid 2 was prepared in a method similar to
the method to prepare the aqueous dispersion liquid 1, and then spherical
mother toner particles 2 were obtained in a method similar to the method
performed in the example 1. A toner 2 was prepared in a method similar to
the method performed in the example 1 except that BONTRON E-89, also
manufactured by Orient Chemical Industries Co., Ltd., was used as the
charge controller instead of BONTRON E-84.

[0271]The toner 2 has an average dispersion particle diameter of 0.25
μm, and a ratio of particles having a diameter of 0.5 μm or greater
was 1.0% by number.

EXAMPLE 3

[0272]In example 3, a prepolymer was prepared as follows: In a reaction
tank provided with a cooling pipe, an agitator, and a nitrogen
introduction pipe, 755 parts of bisphenol A with 2 moles of ethylene
oxide and 195 parts of isophthalic acid, 15 parts of terephthalic acid,
and 4 parts of dibutyl tin oxide were reacted with each other for 8 hours
at a temperature of 220° C., under normal pressure, and were
further reacted for 5 hours while being dehydrated under a reduced
pressure within a range of from 50 mmHg to 100 mmHg. Then, the mixture
was cooled to a temperature of 160° C. and further reacted for 2
hours with 10 parts of phthalic anhydride added thereto. The mixture was
then cooled to a temperature of 80° C., and further reacted with
170 parts of isophorone diisocyanate in ethyl acetate for 2 hours, and
thus an isocyanate group containing prepolymer 3 having a weight average
molecular weight (Mw) of 25,000 was obtained.

[0273]In the example 2, a toner was prepared as follows: In a beaker, 15.4
parts of the prepolymer 3, 50 parts of the polyester (a), and 95.2 parts
of ethyl acetate were agitated and solved. Further, 10 parts of carnauba
wax having a molecular weight of 1800, an acid vale of 2.5, and a needle
penetration at 40° C. of 1.5 mm, and 15 parts of the master batch
particles MB1-M, which was prepared in the example 2, were added to the
mixture, and then the mixture was agitated at a temperature of 85°
C. by a TK homomixer at a revolution of 14,000 rpm so as to disperse
uniformly. Further, the mixture was pulverized by using a beads mill at a
temperature of 15° C. for 60 minutes in a wet-pulverization
process, and thus a oil-base dispersion 3 was obtained.

[0274]To prepared an aqueous dispersion liquid 3, 465 parts of
ion-exchanged water, 245 parts of 10% sodium carbonate suspension liquid,
and 0.4 part of dodecylbenzenesulfonate were agitated in a beaker. This
aqueous dispersion liquid 3 was then heated to a temperature of
40° C. The oil-base dispersion 3 was added thereto while this
aqueous dispersion liquid 3 was agitated at a revolution of 12,000 rpm by
a TK homomixer, and the mixture was agitated for 10 minutes. Further, 2.7
parts of the ketimine compound 1 was reacted with the mixture. After the
solvent was removed from the mixture at a temperature of 40° C.
within 1 hour after the reaction, the dispersion was then filtered,
washed, and dried in a method similar to the method performed in the
example 2, and thus spherical mother toner particles 3 were obtained.

[0275]A toner 3 was prepared in a method similar to the method performed
in the example 1 except that the mother toner particles 3 were used. The
toner 3 has an average dispersion particle diameter of 0.15 μm, and a
ratio of particles having a diameter of 0.5 μm or greater was 3.0% by
number.

COMPARATIVE EXAMPLE 1

[0276]A comparative toner binder was prepared as follows: Using 2 parts of
dibutyl tin oxide as a catalyst, 354 parts of bisphenol A with 2 moles of
ethylene oxide, and 166 parts of isophthalic acid were polycondensed. The
comparative toner binder 11 had a glass-transition temperature (Tg) of
57° C.

[0277]A comparative toner was prepared as follows: In a beaker, 100 parts
of the comparison toner binders 11, 200 parts of ethyl acetate solution,
4 parts of copper phthalocyanine blue pigment, and 5 parts of the rice
wax that was used in the example 1 were agitated by a TK homomixer at a
revolution of 12,000 rpm at a temperature of 50° C., and thus a
comparative dispersion liquid 11 was prepared. Except that the
comparative dispersion liquid 11 was used, a comparative toner 11 was
prepared in a method similar to the method used in the example 1. The
comparative toner 11 had a volume average particle diameter of 6 μm. A
pigment type colorant included in the comparative toner 11 had an average
dispersion particle diameter of 0.70 μm, and a ratio of its particles
having a diameter of 0.7 μm or greater was 35% by number.

COMPARATIVE EXAMPLE 2

[0278]A comparative toner binder was prepared as follows: In a reaction
tank provided with a cooling pipe, agitator, and a nitrogen induction
pipe, 343 parts of bisphenol A with 2 moles of ethylene oxide, 166 parts
of isophthalic acid, and 2 parts of dibutyl tin oxide were reacted with
each other at a temperature of 230° C. under a normal pressure for
8 hours, and further reacted under a reduced pressure within a range of
from 10 mmHg to 15 mmHg for 5 hours. The mixture was then cooled to a
temperature of 80° C., and reacted with 14 parts of toluene
diisocyanate in toluene at a temperature of 110° C. for 5 hours.
Solvent was removed from the mixture, and thus urethane-modified
polyester having a peak-top molecular weight of 7,000 was obtained.

[0279]Further, similarly to the method performed in the example 1, 363
parts of bisphenol A with 2 moles of ethylene oxide, and 166 parts of
isophthalic acid were polycondensed, and thus unmodified polyester having
a peak molecular weight of 3,800 and an acid value of 7 was obtained.

[0280]In toluene, 350 parts of the urethane-modified polyester and 650
parts of the unmodified polyester were dissolved and mixed, and then
solvent is removed from the mixture, and thus a comparative toner binder
mother particles 12 were prepared. The comparative toner binder 12 had a
glass-transition temperature (Tg) of 58° C.

[0281]A comparative toner was prepared as follows: Into 100 parts of the
comparison toner binders 12, 10 parts of the master batch particles
(MB1-M) and 10 parts of carnauba wax that were used in the example 2 were
added. After being mixed preliminarily by a Henschel mixer, the mixture
was kneaded by a continuous kneader. The mixture was then pulverized by a
jet pulverizer and classified by a airstream classifier, and thus toner
particles having a volume average particle diameter of 6 μm.

[0282]With 100 parts of the toner particles, 0.5 part of hydrophobic
silica and 0.5 part of hydrophobized titanium oxide were mixed by a
henschel mixer, and thus a comparative toner 12 was prepared. A pigment
type colorant included in the comparative toner 12 had an average
dispersion particle diameter of 0.70 μm, and a ratio of the particles
having a diameter of 0.5 μm or greater was 15.0% by number.

[0285]The glass-transition temperature (Tg) were measured as follows: A
differential scanning calorimetric system (TG-DSC), TAS-100 from RIGAKU
Corp., was used in glass-transition temperature measurement. About 10 mg
of a sample is put in an aluminum container, and the aluminum container
was loaded on a holder unit, which was set in an electric oven. After the
sample was heated in the oven from a room temperature to 150° C.
at a temperature raising speed of 10° C./min and kept at this
temperature for 10 minutes, the sample was cooled and kept at the room
temperature for 10 minutes. The sample was heated again in a nitrogen
atmosphere to a temperature of 150° C. at a temperature raising
speed of 10° C./min, and then (DSC) differential scanning
calorimetric measurement (DSC) of the sample was performed. The
glass-transition temperature (Tg) was determined from a contact point
between a heat absorption curve around the Tg and a base line using an
analyzer in the TAS-100 system.

[0286]Acid value was measured according to a method specified in JIS
K0070. However, when the sample was not dissolved in the solvent, dioxane
or tetrahydrofuran, etc., was used.

[0287]To determined fluidity, powder density (g/ml) of the toners were
measured using a Hosokawa Micron powder tester. As higher its fluidity
is, the greater powder density the toner has. The evaluations are on the
following four scales of:

[0288]Bad: powder density of less than 0.25 g/ml, Average: powder density
within a range of from 0.25 g/ml to 0.30 g/ml

[0289]Good: powder density within a range of from 0.30 g/ml to 0.35 g/ml

[0290]Excellent: density of greater than 0.35 g/ml

[0291]A lowest fixable temperature evaluation was performed as follows: A
Ricoh MF-200 copier that uses a Teflon fixing roller was used and its
fixing part was modified. In the copier, Ricoh paper 6200 was set and
copying test was performed with different fixing temperatures. The
resultant fixed images were rubbed with a pad, and a remaining rate of
the image density (remaining image density rate) was measured. A fixing
temperature was determined as a temperature with which a fixed image with
a remaining image density rate of 70% or greater.

[0292]To determine a hot offset occurrence temperature, a copying test
similar to the test performed in the lowest fixable temperature
evaluation described above. The resultant fixed images were evaluated
visually for hot offset, and a temperature of the fixing roller at which
hot offset occurred was determined as the hot offset occurrence
temperature.

[0293]To evaluate a gloss development temperature, a coping test was
performed using a fixer of a commercial Ricoh PRETER550 copier. A
temperature of its fixing roller at which images with a gloss of 10% or
higher was determined as the gloss development temperature.

[0294]Haze was evaluated using HGM-2DP type of a direct-reading computer.

[0295]As it is apparent from the results shown in FIG. 11, the toners
according to examples 1 through 3 can produce images in which image
quality and fineness, and low temperature fixing properties and hot
offset properties are balanced. By using these toners, the image forming
apparatus according to the illustrative embodiment of the present
invention can produced images that excel in transparency and saturation,
and the gloss of such images can be controlled. Further, the toner
according to the illustrative embodiment of the present invention excels
in charge stability and color reproducibility.

[0296]Numerous additional modifications and variations are possible in
light of the above teachings. It is therefore to be understood that,
within the scope of the appended claims, the disclosure of this patent
specification may be practiced otherwise than as specifically described
herein.